sunglasses
If you can organise it, it's well worth getting everyone in the class to put on a pair of sunglasses while the teacher is looking at the blackboard. Top hats and red contact lenses is even better. You could also give the teacher a tour of the world, if you have plenty of room under your desks. A string of onions and a beret one time, a three cornered hat and a bockwurst the time after. I would challenge any teacher to get genuinely angry at that.
written by Jo* Blyt*, approved by Mansh
Mrs Waterhouse once taught an entire French lesson without saying a single word about the fact that the whole class were wearing paper bags with little eyeholes cut in them and smiley faces drawn on.
written by an*ny*ou* us*r, approved by Matt