The answer is "elementary". The principal and administrators might control the students and faculty, but the janitors and school maintenance department run the school property. They usually have a work area that all students and most faculty are locked out of. And they have all sorts of storage areas around the school. First thing you need to do after starting work at a new school is to meet some of those guys in the school maintenance department. Find out if there is anything they need to make their job a little better, and see if you can do something about it. If you do them a favor, they might do you a favor and provide a good secure place to keep your bike.