Your school has decide to block Bebo for it's own reasons. You could discuss this with your school putting forward your reasons why they should not do that - good luck.
Should you consider that this is not an option, bear this in mind. Some companies in business block certain web sites for business reasons and consider any employee that tries to negate that as being guilty of Serious Misconduct and therefore open to dismissal.
In other words, you could use this an opportunity to enter the real world.
t, November 2007