If you're stock in the game, here are the instructions...
In the Kitchen...
You must find the elevator to upstairs...
Then... You must find the key in the library...
(Clic to view the key location)
Then... Open the Toilets door (In the library also) with the key...
Then... Find the Principal Door key in the Toilets room...
(Clic to view the key location)
Then run to the Principal Office (At the entrance of the school) and open it...
