Draw, take off my glasses and position them so when I look through them the teacher's head is really little compared to his body(Only do this in Math), doze off, talk, get yelled at, daydream, imagine that there is a little person walking on my desk, imagine things eating the teacher, imagine strange stuff, make weird noises, go into la-la land, and more.
Get on my cellphone, write song lyrics, tap along to songs in my head with my pen/pencil, wander the halls aimlessly or go visit with a more fun teacher's class, write notes to the people beside me, read, talk to the kids in the class... pretty much anything.