That's absolutely rediculous. Have one of your parents (your Mom, I'd guess) call the principal and explain that she is supportive of you riding, and that all you need is a good place to lock your bike for the day. If the Principal gives her any guff, then go above the principal's head and call the school district and ask to talk to somebody who can help. My kids are in GRADE school and they have places to park their bikes, and they use it whenever possible, especially in the summer when its nice out and riding is more enjoyable for them.

All that being said, just find a chain link fence or a light post as somebody suggested, and start riding and lock agains the post or the fence. I can't see it being that big of a deal. If they give you a bad time, talk to some local reporters. Let them know that your school is hassling you because you ride to school instead of walk or bus.