the funniest thing I ever did at my old school was, use remoteshutdown and set it looping every 20 minutes on a hidden machine.

had the network shutting down for over 3/4 of the day, thats until they decided to pull the plug on every switch.

also changing the bootscreens to custom photoshopped, "Youve been jacked by EZ64" was funny to a entire classroom.