Originally Posted by JeffS

I'd start by befriending the janitor, the coach, the music teacher, or any of the other people who have access to locked rooms.

My second thought is a bike locker of sorts. Build it just large enough to roll your bike into the end. Stack books on it, sit a bookshelf in front of it, whatever - so that it blends into the room. Doesn't take up less space, but becomes less of a problem.