a few years back we lined our entire school bus with a thick 2-3inch coating of shaving cream. on the inside. bus driver stopped caring after she looked back to 8 or so kids spraying it everywhere.
also a senior class back in the day before lazer mice, took all the little balls in computer mice and put them in some girl's locker, which she never used (she didnt know). they replaced all the balls over the summer, then 1st day of the next year some freshmen opened his new locker, and was rained on by rubber hard balls.