They don't have to provide parking unless there is some contract somewhere that compels them to provide it. Remember, entities have an almost unlimted ability to form contracts.
The OP should try to find anything that may be construed to compel the school to provide parking. Something that comes to mind is that some schools sell parking permits that are valid for the year. Such a permit, especially if you have paid money to obtain it creates either an explicit (if you signed something) or at least an implied contract for the shool to allow you to park there. If all of a sudden they stop providing this service, they are in breach of contract.
Having said that, unless you want to take them to court - and you will graduate before it comes to trial, then you should work out an amicable solution to your problem.
Some suggestions include finding out who is defacing the school property. Set up some night vision cameras at some distance and leave them recording. Hard disks and computers that can do this are cheaper now. A few days of sureilance might show that it could be a rival school that is doing it. If so, call the cops - present your proof and I bet the problem is solved.
Work WITH your principal and not against him/her. Go to his office, and set up a meeting with him and yourself. Involve your school student government if you have one. Show him you want to work towards the common goal of getting yuor parking back or keeping it.
If you do this, I'll bet he tries to work with you. it will show good initiative. He will want to solve the problem as well.
Hope that helps.