and I remember one that still gives me giggling fits!

our football field had porta potties for the players and guests during games.
anyhow we mounted a small portable radio in one and tuned it to a dead frequency, we had wireless microphones transmitting the same frequency and would watch when someone entered.
after a minute we would make smart remarks and comments to see how rattled they would get.
rattled the teachers more than the students!

more than a few ran out with their drawers down yet