At my school there was the dead fish in the vents one year.
We had flags from many countires hung throughout the school.... one year some kids stole them all and replaced them with perverted.. don't remember what
At another high school in my city a group of kids got a chopper(dont as how) and air lifted the Mcdonald's playground onto the roof. And everyone was sliding down the board and playing in it when school started... that was great.
We also had a huge ramp in my school that everyone used to run down. Kids talked about putting baby oil on it one year..... but thank God they didn't
Some kids did superglue the stairs one year though.
There is also this infamous one. Break in at night and switch out all of the teachers desk drawers. Just mix them up all around the school. Of course this means they gotta have the same desk, but many do.
Our high school mascot was a knight. The year I was a junior, the seniors stole our mascot...it was an imported knight from Spain worth around 3000 bucks - one of those steel plated setups with the armor, shield, and sword. They broke the display case which was around another 1000 bucks and rumor has it, spread it's parts all around the county. Nobody found out who did it, but there were people who were rumored to know. The next day the principle said we had screwed ourselves over and trashed our own mascot. It was kinda funny.
Too cool for school.