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.