at school, the usual loosening of the salt cellar tops in the dining halls, putting condom wrappers in teachers blazer pockets.
also phoning in bomb scares to our (mixed religion) school (worked a treat in Northern Ireland in the 80's lol)
i once persuaded my 5yr old little brother that touching electric cow fencing with his knob would make him magic, the poor sod, it nearly gave him a heart attack lol.