If you're bold and/or just want to start trouble (I'm in no way saying you should, just throwing this out there), lock it to what ever you can near the office and then tell all your friends to ride their bikes and do the same. Make it seen and maybe school policy will change.

I think there is some kind of legal responsibilites for students leaving schools, your prinicipal might be telling you the truth. It could very well be the whole trip between home and school, or until you are with a parent.