Basically your school has chosen to block the site with a firewall or similar filtering device. You will not be able to get to the site directly and therefore need to use another site (known as a proxy) to get the site for you. Because you are going to the address of the proxy and not myspace the firewall will not block you.

If you start sending allot of traffic to one of these proxy servers they sometimes raise flags and will also get blocked, never mind there are thousands to choose from :)

Most of these services are free and funded by advertising, they are anonymous and do not track any of your browsing habits or passwords and are quite safe to use.