Some general guidelines for a good prank:
-Don't do anything they would get you in a stuffload of trouble if you got busted. This eliminates both suggestions (liquid spray and sugar free gummy bears)
-Don't do anything that causes pain. Those sugar free gummy bears are nasty and can cause some pretty bad gut pain so that's out.
-Don't do anything that affects people other than your target. The spray will disrupt entire classrooms, so that's out