Extra shields
There are many "shields" on the wall around the school . Press J to jump and collect them. Some of them are too high to reach normally (you cannot hit them with a fist) -- catapult on teachers to get them.
Bombs
There are also bombs in the fireplace room (on the right in the first floor), where the Caretaker Albert (man who always say "Bloody kids!" when you hit him with catapult) sometimes opens the door. You can sneak inside the room and take one of the bombs by pressing C. Press B to use the bomb.