My work banned myspace for many legitimate reasons...but we were able to get into it on one of our computers (in the office, away from customers) by going into the "restricted websites" area and allowing/unblocking Myspace. However, once corporate found out, they went in from our moderator and locked it out while putting a password on it. Your school may have done something similar. On every computer there is a setting for a main user or "administrator". The only way to bypass a block set up by the administrator would be to hack in as the administrator with the password to the computer...this unlocks the complete system. DO NOT DO THIS. It is illegal and if you get caught, believe me, there are severe consequences. Myspace should be used on personal time and not in a school/work setting. Sorry....I hated it at first, but there are too many things that can happen to computers (viruses, porn bot downloads, etc....) and unless you want to pay to have them all rebuilt...or worse, be suspended or expelled, don't do it.
Try petitioning to have a computer set aside -in the computer lab or wherever- for students to have access to personal sites. The students can reserve a spot and have a set amount of time to check what they need to check, and even sign a statement that they will pay for repairs when any computer is damaged. It might work.
****Update****If you USE the guy beneath me's idea, don't do it at school or you are REALLY SCREWED. Those sites may already be blocked as well...and the administrator can see everything you are doing. They can even log into your computer while you are on it by your IP address...and watch every move you make. If they choose, they can even shut down your computer and completely revoke your privileges...I hope I am a help!!!