Everyone's always telling you what to do while at school. Your parents tell you to attend, your school staff members tell you what to learn, what to eat, what to wear, who to be friends with... the list goes on and on. It's a lot like prison, except that you didn't commit a crime. You'd expect that after coercing you to attend school and all these things, they'd at the very least allow you to browse whichever web sites you'd like while you are there. Apparently not. The interwebs are censored, a great many of web sites are being blocked. Even so, there are a couple of ways to get past the restrictions, and it's a lot easier than convincing your parents to homeschool you.
An unblocker is like a gateway that sit between your computer and the site you're trying to visit. The unblocker goes to the site on your behalf, downloads the page, and forwards it to you. So the site you want to be on never sees your IP. So actually you are visiting sites in an indirect manner. School firewalls think you are just accessing the unblocker, not the site you actually want.
An alternative method of unblocking Facebook in school is to set up your own unblocker tool. This is more complicated than using someone else's unblocker but it pays off because you're in control. Explaining how to do this is not in this guide, if you search for "How to make your own proxy" you should find instructions. You can also make some money from this by putting ads on it.
Other ways to unblock Facebook include to use sites that translate pages, for example Google Translate or Babelfish. Also try Google's cached version of the site. However, you can't to login that way, so it's still best to use an unblocker for Facebook, though these tips are good with many other sites.