Seems like your school dont want you using Myspace ! hah,
There are 2 ways to usually get around schools very cheap security measures.
Using a proxy server to get around the block (quite advanced)
Or for an easier time, try using a website anonymiser. basically this website enables you to surf the internet on most pages, whilst the anonymiser tricks the internet and the security measures by showing that you are on a legal unblocked website.
their are a few Anonymisers that are useable, whether the school security blocks them or not is unknown here are a few :