The bell clapper to repair the school bell is hanging near the ceiling. Once the seagull is run off, you can repair the bell with it.
(see related question)
The clapper.
by the green cabnit and what is useful in the secret room is the bell clapper
In Nate's locker. The combination is on the assembled comic strip.
in the cabinet on big Nate island
It is in the secret store room behind the filing cabinet in the detention room on Big Nate Island. You need the blue prints from Nate's locker in order to access the secret room.
you climb up the rope
It it in the secret passage in the detention room on Big Nate Island.
You have to get in detension
you use the stink bomb wow
in the detention room
get the bell clapper
pull the power switch on