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.