Cum for Class

"There you are! What took you so long?"

My sister was leaning against the wall in the corridor, looking stressed. I guessed it had something to do with her project - the university professors apparently kept their master's degree students busy. Things were less hectic for us first-years - we could work really hard if we wanted to, but nobody was breathing down our necks like teachers did in high school, to make sure we did our homework. Having been at the university for about four months now, I had found a nice work rhythm that made sure I kept up, while still giving me time for hobbies and relaxation. I think my sister was working quite a lot harder than I was.

"Hey, sis. Got your message..."

"Yeah, that I sent two hours ago. And an hour ago. And half an hour ago. Where were you?"

"I had things to do - am I your servant now?"

"Yes! Well, at least... look, you agreed to help me with my lab project. You know the one that I'm right in the middle of?"

"That explains the ponytail."

My sister had long and wavy blonde hair that usually filled up a room when she let it loose. Wearing it in such a tight ponytail caused it to be pulled back, so she could keep it away from her face and put it down the inside of her clothes. She didn't usually wear it like this unless she really needed to keep her big mane in check, so I had guessed that she was working in the lab the second I saw her.

"You guessed I was in the lab because I have my hair like this? The lab coat didn't tip you off?"

"Oh... yeah..."

I actually hadn't noticed. People were wearing those white things all over the campus building, so I didn't really see a direct connection with lab work.

"Well, your hair looks great when you wear it loose, so..." I said, trying to get out of the doghouse with a bit of flattery. I knew she liked her hair. And it did look great.

"Don't change the subject. You agreed. Look, you're an adult now, you can't just make appointments and then blow them off. This project is important to me. I'll be presenting the results in class! On Monday! In front of the professors and everybody! I have to finish the lab work today if I'm going to have time to file the report before Friday. I have two hours of lab time left. After that, the microscopes are booked by the plant physiology course."

She looked at her watch and then back at me.

"Okay, I'm sorry. What can I do? You mentioned something about cell samples...?"

"Get in here."

She grabbed my shirt and pulled me in through a door nearby. Once inside, I realized it was a restroom.

"What are we doing in here?"

"I need a sperm sample."

"Uh... whaaa...?"

My sister produced a glass test tube from inside her coat. It was sealed with a plastic cap in order to prevent the contents from getting polluted. Right now, though, it was empty. Sis started talking quickly, clearly in a hurry.

"For the next stage in my project, I need to measure the motility and cell count of human spermatozoa under different conditions. I'm going to change variables like temperature, enzymes and pH in order to study the effects."

"B-but..." I stuttered nervously.

"Don't worry, you'll be anonymous. Nobody asks who the donors are anyway."

"But..."

"We often use male students for this, but there are only five guys taking the course this year, and they're rarely around, so I figured it'd be easier to just get you to do it."

"Yeah but... how do I..."

"Jesus, you really are clueless... You know how to make yourself cum, right? Just take this cap off and make sure you get it in the tube when you're ready to blow. Try not to spill any. The more I get, the better."

I looked around. It was pretty public, this place. It was a nice and clean restroom with several stalls, but even though it was empty at the moment, the corridor outside was full of faculty members and students walking by. I could actually hear the distant chatter through the door - talking, laughing, moving around... and there was no lock on the outer door.

"But ... can I do it ... here?"

"Oh, I'm sorry, not good enough for Your Highness? Well, I was gonna find somewhere a bit more private, and take my time briefing you about the procedure and everything, but you blew that plan out of the water by being two hours late! Now get in there and do it; you jerked off at least twice a day when we were living at home."

"You knew about that?"

"Hah. I could hear you through the wall. Look, it's easy, just think about some hot blonde, or that redhead you were staring at last week - she was pretty busty, right? Think about doing stuff with her. Dirty stuff. Mouth stuff. Butt stuff. Now go."

She turned me around and pushed me into a cubicle. The door closed behind me. I neglected to lock it, as I was still kind of shocked at this whole thing. I just stood there with the glass tube in my hand and tried to think straight. Sis was right, though, I had sort of blown her off when I saw the first message alert. Figured she could wait while I got some lunch. I had completely forgotten that I'd promised to help with her project, although I didn't know what the project was at that point. Still... there would probably have been more time to get comfortable if I hadn't wasted so much time. So yeah, she was now in a hurry and was probably justified in putting some of that blame on me.

I pulled down my pants. Tried to get comfortable. Tried to think about exciting things that would give me an erection. The pressure of the situation didn't help, though. The time pressure, my sister's annoyance with me, the public place we were in... I tried thinking about Beth, the beautiful redheaded girl in my botany class. She had freckles all of her face and a really nice rack that I always found it hard to keep my eyes off. I wondered what her boobs would feel like to touch. Imagined myself grabbing hold of them, touching and squeezing... I could feel my cock starting to respond. Then somebody came in.

I heard the sound of the outer door opening, and a woman's voice saying:

"Good afternoon, miss Johnson."

"Good afternoon, professor Williams." my sister replied.

Then I heard the sound of somebody getting in the cubicle next to me. It occurred to me that this was clearly the women's restroom. As if I hadn't had trouble getting hard already, now there was another thing serving to distract me. My dick started getting limp as I tried jerking it. Eventually, though, I knew I had to wait until the other lady had left the stall next to me.

Seconds ticked by. Possibly minutes - I didn't have a watch. It felt like minutes. At last, the lady flushed the toilet and left to wash her hands outside. I hoped nobody else would come in. If left undisturbed, I might be able to rub one out in a few minutes' time. As I waited, I could hear her exchange pleasantries with my sister out there. I almost blurted out:

"Oh, come on! Give me a break here, go to the cafeteria if you want a chat!"

Eventually, though, she left. Leaving me alone again, with my sister guarding the door to my stall.

"How's it going, bro? Done yet?"

"N-no... not exactly."

"What do you mean 'not exactly'? It's not rocket science or anything, just ... hurry it up!"

"Yeah... just don't pressure me, all right?"

"All right..."

Another couple of minutes went by. I tried thinking about Beth, movie stars and hot models. Nothing helped. I couldn't get properly hard, let alone cum; the time pressure and thinking about where I was and that someone might walk in... it all just kept interfering. Eventually, sis nagged at me again.

"Are you done yet? Come on, what's taking so long?"

"I... I'm sorry... I just..."

I was starting to feel ashamed. Like I was failing in my manly duties, unable to satisfy a girl in the sack. Then another thing happened that caused my half-limp erection to fade even more. Sis popped her head in.

"What's going on?"

"Jesus! Sis, get out of the..."

"Look, there's no one else here, and I've seen your wiener before."

"Yeah, when I was ... like, five."

"Dude, you got nothing to worry about, it's fine. I just... really need you to finish ... quickly."

She looked even more stressed that I did. Eyes flickering this way and that, toes tapping, body fidgeting. She looked at her watch every twenty seconds or so while looking like she was trying to come up with solutions to a problem on the fly.

"I'm sorry, sis, it's just hard with all this pressure... it takes time, okay..."

"But time is what I don't have! I can't do lab work in the evening like I did last time, professor Gregorovich said they'll be locking the lab after some idiots started messing with his equipment..."

I stood there, facing her, feeling stressed out and ashamed. Pants around my ankles. My limp dick hanging down between my legs. Sis looked down at it, then back up at me, then at her watch. Suddenly, she got a serious look in her eye. With a determined tone in her voice, she said:

"Fuck it. I don't have time for this."

She stepped inside the stall with me, locked the door and pushed her way past me so she could sit on the toilet. She bent down slightly, and before I knew what was going on, my whole dick was inside her mouth. My eyes immediately went hazy as the feeling of her flickering tongue hit the pleasure center in my brain. As she licked the sensitive head, I started to harden. She then started moving her head back and forth, creating suction with her mouth and taking me in and out as I grew harder and harder. I was still nervous as hell, but the feeling of her warm, wet mouth gliding up and down the whole shaft was just too much. My cock went fully erect in seconds.

"Jesus, sis... the fuck..."

"Mggllph... just... hold that."

I had the test tube in my hand, still with the cap on, and she was anxious for me not to drop it. She started using both hands; wrapping her fingers around my shaft with one of them and gently stroking underneath my scrotum with the other. I started moaning louder and louder, forgetting where we were and that anybody who came in would be likely to hear me.

"Ffff... fuuuuck..."

I couldn't believe what was happening. My heart was pounding in my chest, sweat started beading on my forehead and I could feel the hairs at the back of my neck standing up. I had known for some time that my sister was a bit of a slut during her high school days, but nothing had prepared me for the vision of seeing her sucking my cock in a public restroom. But there she was, sitting in front of me, giving her little brother the most amazing blowjob I had ever experienced.

My head started spinning as she picked up the pace. I couldn't resist putting my hands behind her head. Moving my hips forward gently, it felt like I was fucking her mouth all the way down her throat. She started making loud slurping noises and the sensation was getting more and more intense. I looked down at my sister and couldn't believe how hot she looked, doing this to me while dressed up in all her science gear like the lab tech she was.

"Oh my God, sis... you look so fucking hot..."

"Mmpphsssllrrpp... sssllllrrrpp..." she kept slurping while bobbing her head backwards and forwards in long, drawn-out motions, as if slowly headbutting me in the groin.

We went from naught to sixty in no time at all. Having been limp and nervous mere minutes ago, my amazing sister made me so horny that I was having trouble staying on my feet, and had to lean back towards the door to avoid falling over. I was getting a tingling feeling between my legs as I held my hands against the walls and bucked my hips forward.

We were both moaning now - clearly, I wasn't the only one getting excited. I thought back to those wild times when our parents were away and my teen sister brought boys home; the sounds of her moans and orgasmic screams still echoed in my ears. My sister loved sex, all kinds of sex, the dirtier the better. She wasn't the teenage slut anymore, she was a respectable woman doing important science experiments in her lab coat... but she was also sucking her brother's cock in a bathroom stall. The thought of how dirty this whole thing was just made me forget time and place completely. All nervousness melted away as the only thing I could think about was that my hot sister was choking on my dick. I was seconds away.

"Sis... I'm ..."

"Yeah? Ready to blow? That's what I want, give it to me..."

She quickly took out some kind of tissue from her coat and wiped the head of my cock with it. Then she started jacking me off -- she wrapped her hand gently around the slippery shaft and started sliding it quickly up and down without touching the sensitive head. The feeling grew more and more intense, building up until my mind was spinning like a whirlwind. I could feel the glass tube being taken from my hand, but at this point the only thing on my mind was what was going on downstairs. She held out her thumb so that it gently stroked the underside of the glans as she jacked me off. It was intense. I couldn't believe I hadn't cum already. My legs were literally shaking. It was like she was controlling exactly what was happening to me, building up my orgasm until it was as powerful as she wanted it to be.

"I'm... gonna..." I gasped as I felt the pressure continuing to rise.

"Yeah... Cum for me. Cum for me, baby brother..." she whispered.

"I'm ... I'm gonna..."

"That's it. Cum. Cum for your sister."

That did it. I moaned loudly as I shot stream after stream of cum into the glass tube. She held the tube firmly in front of the hole with one hand and kept stroking up and down the length of my cock with the other, giving me such an intense orgasm that I could feel my legs buckling under me.

"Aaa-aaaaah... sis! I ... aaaah..."

"Keep cumming, come on... keep going, keep going..." she kept coaxing me.

After the first thick ropes had shot into the glass, it kept coming out of me in big droplets that my sister eyed longingly. I got the feeling that she would have loved to simply put her mouth back on me and swallow it all. Instead, she stroked the underside, her finger pressing against my urethra to milk every single drop of semen from my pulsating cock.

"Oh... aaah... sh... shit..." I kept moaning and gasping, still dizzy and shaking from what felt like an overdose of pleasure. Sis kept stroking my cock until she was sure I had given her every drop.

"Nice goin', bro." she said and looked at the white goo in the glass tube while expertly securing the cap to seal it shut. "I hope there was no contamination from the saliva... Pretty big load you got, though. It'll definitely be enough for my tests. Thanks."

"Thanks... you too."

"Oh, by the way.." she said as she stood up and started sneaking past me. "Tell anybody this happened, and I'll snap you like a twig, got it?"

"Yeh..." I agreed, still a bit dizzy.

"Anyway, I really gotta go. Get this processed. Seeya!"

Sis placed the tube in her coat pocket and made her way out, making a quick stop at the sink to wash herself off before leaving. And then, just like that, she was gone. I pulled up my pants and staggered out of the stall, splashing a bit of water on my face before going out into the corridor. Unlike sis, who was busying herself with work, I would definitely need to take the rest of the day off.