in the headmasters office there is a desk immediately to the right of the door, next to that is a datacube on the floor with a login for the computer behind the receptionist.
if you want to go through the door that has a keypad behind a plant, then i think the keycode is with the templars above nassif's apartment, or you can go through the vent in the room with the greasels
you can either get the code of nassif in north medina, although if he doesn't like you (if you destroyed the greenhouse), you'll need to get the code out of a box in the room behind him using a multi tool