Everyone's constantly ordering you around while in school. Parents and teachers are telling you what you ought to think, who you can spend time with, what you should wear, and even what you ought to eat. It's a long list. It's a lot as if you are being treated like a criminal or something. You would expect that while forcing you to go to school and all these things, they'd at the very least have the decency to let you visit any websites you will need while you're imprisoned there. Apparently not. The internets are censored, and a good many of sites are blocked. Even so, there are many ways to go around the restrictions, and it's a lot easier than getting your parents to homeschool you.
Unblockers are like a gateway that sit between the computer you're on and the site you want to go to. The unblocker accesses the site instead of you, loads the page, and sends it off to you. So the site you're trying to visit never sees your IP. So, basically you are surfing websites in an indirect way. School filters only see you accessing the unblocker, not the site you actually want.
Another way of unblocking Facebook at school is to set up your own unblocker site. This is more advanced than just using an unblocker but better in the long run 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 will find info. You can even make money from letting people use it if you put ads on it.
Some other ways to unblock Facebook are to use sites that translate pages, such as Google Translate or Babelfish. Also try Google's cached version of the site. Although, it won't let you to login using these methods, so it's still best to use unblockers for Facebook, however these tricks work well for many other sites.