I'll start things off.
This happened during my junior year in highschool. See, due to the way class schedules were drawn up, we had an extra fifteen minutes than usual. The principal decided to institute a "break" period for students. This happened around mid-morning between first and second period. Well, for first period I had biology. April fools day rolled around, and I decided to have some fun. I knew the teacher for a long time, so I knew good well that he'd be able to take a good prank. I slipped a can of shaving gel into my backpack before leaving home. Right when the bell rang for break, everyone made their usual mad dash for their usual hangouts. Even the teacher was gone, as usual. I made sure the coast was clear, and decided to go to town. I sprayed some over portions of the blackboard (this board was made for markers, not chalk so it wouldn't do anything to impair writing). I even sprayed (and later lathered) "HI!". I took a palm full and lathered it under the lip of his chair, so he'd get it on his hand when he pulled his chair out. Then I used the last of it to lather up the first couple layers of paper towels.
However, right when I was about finished, a couple of girls walked in and caught me doing it. They started laughing, dropped their stuff off and walked out. 
Right at the end of the day, I made my way for the exit by the gym. Well, he's also the basketball coach, so as I was walking by he happened to spot me. I assume the girls snitched on me since he knew good and well who did it.
He yelled my name, and told me that he was going to get me good for that. 
