At school, everyone forever wants to boss you around. Your parents tell you to go, the teachers tell you what to think, what to eat, what to wear, who to go out with... the list goes on and on. It's almost as if you're a prisoner or something like that. You'd expect that after forcing you to endure class and all these things, they'd at the very least let you visit any web sites you will like to as long as you're there. Apparently not. The interwebs are censored, many sites are blocked. Even so, there are many ways to unblock websites at least, and it's a great deal more easy than getting your parents to homeschool you.
An unblocker is a kind of gateway that sit between your computer and the site you're visiting. The unblocker goes to the site you want instead of you, loads the page, and sends it off to you. So the site you want to be on only sees the unblocker's IP. So actually you are browsing sites in an indirect manner. School firewalls only see you visiting the unblocker, not the real site.
Another method to unblock Facebook at school is to make your own unblocker site. This is harder than using an unblocker but it's a lot nicer to have because if they block it, you can make another. Explaining how to do this is too much effort to explain here, do a search for "How to make your own proxy" you should find instructions. You can also earn money from your unblocker by putting ads on it.
Some other ways to unblock Facebook include to use sites that translate pages, for example Google Translate or Babelfish. You can also try using Google's cache copy of the site. However, it's not possible to login using these methods, so it's still better to use unblockers for Facebook, though these tips are good with many other sites.