My favorite for lockers was to use clear caulk arounf the door edges and vents, then siphon 5 gallons of water into it from the top vent.
Reminds me of a college prank we used to do. Fill 55 gal trash can and tip it against the door to someone's room, then bang on the door. When they yank the door open the deluge begins.
Another one was to take the same can filled with water and dump it down the back stairwell as someone was coming up. You had to wait till they got to the middle landing to ensure proper soakage.