We put clear chemical indicators in the urinals. One young freshman came running out, screaming "I'm peeing blue!"

Another time we put a piece of rubber balloon over the end of the mouthpiece of the Sousaphone. This made is VERY difficult to get any sound out of the thing and drove the band director nuts.

Another band trick was to loosed the clamp that held the reed on a clarinet or saxophone mouthpiece. Not a lot, mind you. That too drove the band director over the wall.

Come to think of it, I remember that some guys horsing around in the bandroom after school broke his expanding baton. The next morning, he did a particularily violent motion with it and the front half flew across the room, barely missing the Third Trombonist.