Well if everyone else is talking about bodily function stories.....
Last year around the start of senior year, I had a big breakfast that morning, and we were issued text books that day. I made the mistake of trying to carry them all in my backpack and it caused alot of pressure on my back which in turn caused me to have to shit.
I tried holding it through math class and a friend I was sitting close to noticed me figiting. I told him that I was in dire need of the bathroom and had to sit through my Algebra 2 teacher's ramblings for 15 minutes.
At long last I finally broke free and made my way for the bathroom, without taking the time to close the class room door, and it was directly across the hall.
Somebody had left the seat up in the stall I was in, so in my hurry to shit, I slammed it down causing a huge noise to echo through the halls.
I'll leave what happened next to your imagination, but when I got back to class, everyone was working but the one dude I spoke of earlier was looking up smiling at me.
When I asked him what was so funny, he responded "SLAM!"
After that day I turned slam into a catch phrase and made it my mission to spread that story throughout the school.
What started as a horrific event ending up being one of the funniest and most re-told stories of my senior year.