Two things come to mind:
1. Me and all the seniors took a shit ton of nachos and smeared it on all the cars of our teachers. Was easily the funnest night we had.
2. On the last day of school, 5 minutes before the last bell, we lit 5 "Beacons" on top of each building, then we blew the ceremonial horn. (Trumpet) We kept the beacons covered with metal and wet down everything around us so nothing would light. We were arrested for attempted arson, but was removed after questioning.