Ah Muck Up day, glad to see it's reknowned all over the world!
Well, put a fish in a doorway, they totally stink out the whole school. Put straw all over the ground and in trees etc. Get some sheep in the school. Put water bombs with flour etc on toilet taps. Ehm, Im trying to remember what else was done...Some people smashed windows, but that's a bit extreme. Put treacle on door handles. Just things like that.