The simplest way to unblock Bebo at school is to use an unblocking website. You can get on any site that is blocked this way. There are links to unblockers at the end of this page.
How does it work?
An unblocker is like a gateway that sit between the computer you're on and the site you want to go to. The unblocker goes to the site you want in your place, loads the page, and sends it on to you. So the site you want to access only sees the unblocker's IP. So actually you are visiting sites in an indirect manner. School filters only see you visiting the unblocker, not the destination site.
Some advice for using unblockers for Bebo
It's better to find an unblocking site that isn't used much. Popular unblockers will be found and blocked by the school. Most unblockers found by doing a search will get blocked more quickly because teachers can find them so easily. Even if you find a good one it would be smart to have a backup you can try if your main one gets blocked.
An alternative way to unblock Bebo at school is to set up your own unblocker tool. This is more complicated than using someone else's unblocker but better in the long run because if they block it, you can make another. Explaining how to do this is beyond the scope of this article, but if you search for "How to make your own proxy" you should find instructions. You can even make money from this if you put ads on it.
Other ways to access Bebo are to use sites that translate pages, for example Google Translate or Babelfish. You can also try Google's cached copy of the site. However, it won't let you to login that way, so it's still better to use unblockers for Bebo, however these tricks work well for most other sites.