In all my years of schooling, the worse I ever did was about drive a teacher insane....

He got me po'ed when I discovered a list in his storeroom of students he felt were drug addicts. My name was on that list and it was total war then. I've never done drugs.
My best prank was the infamous biology film. He made the mistake of mentioning that it was 35mm film and it would be shown the next day.
I went to a friend that was working as a projectionist at a local theater that had a red eye show of porno movies every Friday night starting at 1am. He gave me about 15 minutes of film and showed me how to splice it into the other film.
The class I had was first period and I went into his class before hand and did the dirty deed. When he started the movie. He was sitting beside the projector grading papers by the light from the projector. I had made the splice just after the credits and he wasn't paying attention. The porno part started and you could hear the gasps of the students. He looked up and went totally ballistic....

....he tried to rip the reel off the projector with it running....lmao. He got it turned off and immediately looked at me and said, "YOU! I know you did this!" He then left the room to get the dean. When they returned, the movie was normal.