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.
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.