Two stories stick out in my mind...
1) As a senior prank, someone snuck into the gym, got into one of the ceiling vents, and dropped dead fish, and God knows what else, into a hard to reach area/section of the wall. He then proceeded to close the vent up, and make it appear that nothing had been changed around. Fast forward 7 days, and half the school smells of death. Classes are being held outside/in the lunch room, because some of the rooms smell so bad that people are throwing up. Finally, on the 8th day, school was canceled. A group of police officers search the school because the smell is so bad, teachers are worried it's animal/human remains. The finally locate the source, clean it up, and we all go back to normal classes (but the school had lingering strange smells for another few days). Half the kids knew who did it in the first place, but he got away with it because there was no proof. lol
2) When I was in middle school, a "problem" kid was caught smoking in a bathroom by a pregnant teacher. In his attempt to escape punishment, he knocked her over and she smacked her head pretty bad. He had an in-school suspension for weeks. Teacher and baby were fine, but she had a nice big bruise/lump for a while.