Bagpipes. Always so VERY fun to have go off... In the middle of a test... 25 feet away from you...where one couldn't POSSIBLY reach it in time to turn it off...
Death metal works too, and even better is the Star Wars theme.
For a good laugh, try the Super Mario theme.
For a class-stopper, use something very subtle, that will make people even WONDER if they actually heard something. A loud "beep" would be sufficient, or a deep, long, slow, maniacal creepy laugh.
For a 10/10 in AWESOMENESS...
Have it start playing jokes; but really corny ones. Chicken crossing road stuff, Chuck Norris jokes are EXCELLENT

Just suggestions that won't get him suspended/ you killed.