My senior class planned on having a water balloon fight with the Juniors in the parking lot after school but thanks to a snitch the principal threatened to hold our diplomas if we threw any water balloons.
So, that night 5 seniors broke into the school, got into the student government closet and stole shitloads of glitter and put it EVERYWHERE in the hallways. The place was freaking covered. After they spread all the glitter, they broke in to two classrooms and swapped all the stuff in the two. The next day the janitors tried to vacuum it up before school, but the vacuums didn't pick any of it up and with people walking on it during the school day just mashed it all down into the carpet. The prank was pretty awesome, but the cameras found out who it was and they got charged for breaking and entering, vandalism, and theft.