Water and flour is puerile and lame, and I'm pretty sure I have heard the sheep story at least 3 times of completely different people. One of the best pranks I have ever herd of was directing traffic into the school just before rush hour, causes havoc. Also putting bricks/wood underneath the Head's car axle so that the wheels producing drive are a couple of mm of the ground, so when he tries to accelerate his car will not move, but it looks fine from the outside.