This is a broadening of the Unusual Facts thread (which I enjoyed very much).
Everybody has a couple good stories to tell, let's hear them!
When I was a Junior in High School (~16), I was chatting with friends in our first class of the day before the bell. It was a Current Literature class and my friends Rob, Sylvia, Myra, Lisa, and I were sitting at our desks talking to each across the aisle. Lisa was across from me, and as we talked she would rub her leg with a bemused expression on her face. Finally she said, "What...?", reached up her pant leg and pulled out: a pair of underwear! The look of total shock on her face, followed by a mortified shriek, was priceless. Naturally the rest of us were crying with laughter. We joked about it at the 5 year reunion.
Senior year was fun. Senior pranks were a small tradition at our school, though I bet most of them were myths passed on year to year. But we actually did a couple that I was proud of. I was on the swim team and one evening a couple of the guys took up a collection and bought 120 goldfish. We had taped one of the outside doors for the pool open before we left practice that night, and the next morning they released them into the pool about 7:30 am. The chlorine and lack of oxygen in the pool killed them quickly, so that by the time the morning swimming class arrived, most of them were floating. I saw the Principal a couple hours later (my friend's Dad) and he had a huge grin on his face as he remarked on this astounding occurance, wondering if I had heard about it.

I did my best to look astonished.