[haydnwalker]

Most schools should be running their internet through the NGfL (National Grid for Learning) which provides content filtering through WebSense. This is in theory not possible to bypass as the school itself doesn't control the connection and whichever proxy you use wouldn't allow access to the internet at all (other than the NGfL proxy)

Don't bother - you'll just have your Internet rights revoked (at the least) and you could be kicked out and be arrested for trying to circumvent security as part of the computer misuse act.