I went to an experimental sort of high school. We had a rich and lively hall culture, and we had commandeered one of the science offices (as well as one of the English offices and a social studies office), for hanging out and putting our stuff in.

For some reason, most people kept their musical instruments in the science office. There were usually one or two French horns laying about in the office, along with some bunsen burner hose and a desk that was only a shell and was completely hollow underneath.

One time, when I had a bit of a wild hair up my arse (which really was most of the time), I took one of the French horns out and laid it on top of its open case. I took the mouthpiece off of the horn, attached a length of bunsen burner hose to the French horn, attached the mouthpiece to the other end of the hose, and arranged it all on top of the hollow desk in such a way that the hose could not be seen by people entering the room. It just looked like a French horn placed somewhat carelessly on top of an open French horn case.

Then I sat under the desk, holding the hose with the mouthpiece at the end of it, while another student sat at the desk about two feet away from the French horn. The plan was for him to give me a little kick when someone came into the room, and I would blow on the mouthpiece... no small feat, since it required a lot of air pressure to make a noise with that much bunsen burner hose between me and the horn. We succeeded in startling a few people with the joke, but the best was the guy who was stoned out of his gourd (I don't remember if it was LSD or hashish, but he was wrecked). He walked in, heard the horn blowing all by itself, said "Ohh... My... GOD!!!", turned around, and ran out of the room. I still remember that stoner quite fondly.

When I was in junior high school, I used to like to put spitballs and pour salt down the pants of the guys sitting in front of me as they leaned forward in their seats (creating a bit of a gap between the top of their pants and their backs). That was a lot of fun.