CRIMES ACT 1958 - SECT 72
Basic definition of theft
(1) A person steals if he dishonestly appropriates property belonging to another with the intention of permanently depriving the other of it.
(2) A person who steals is guilty of theft; and "thief" shall be construed accordingly.
Each of the points of proof must be satisfied in order to obtain a conviction for theft. The most difficult will be "dishonestly" because the logbook was arguably appropriated with your consent.
Try the theft approach and make a statement to police if unsuccessful, but you may have to take a civil case to the Magistrates' or District Court in detinue which is otherwise known as trespass to goods.
Moral...don't leave your property with the School regardless of how helpful they are.
Kaz