Everyone's forever telling you what to do while at school. Parents and teachers are insisting on what you should write, who you can and cannot hang out with, what you can wear, and even what you must eat. It's a long list. It's like prison, apart from the fact that you didn't commit a crime. You'd expect that since you are forced to attend class and all these things, they would at the very least be nice enough allow you to visit whichever websites you need as long as you are there. Well, no. The interwebs are censored, many websites are being blocked. As luck would have it, there are many ways to unblock sites, and it's very much more easy than convincing your parents to homeschool you.
An unblocker is a kind of gateway that sit between your computer and the site you're trying to visit. The unblocker accesses the site in your place, downloads the page, and sends it off to you. So the site you want to access never sees your IP. So actually you are visiting sites indirectly. School firewalls only think you are visiting the unblocker, not the site you actually want.
An alternative way to unblock Facebook in school is to make your own unblocker site. This is more advanced than using an unblocker but better in the long run because you can make more of them. How to do this is beyond the scope of this article, if you do a search for "How to make your own proxy" you should find some guides. You can even make some money from letting people use it by putting ads on it.
Other ways to access Facebook include by using translator sites, for example Google Translate or Babelfish. You can also try using Google's cache copy of the site. However, it won't let you to login that way, so it's still best to use unblockers for Facebook, though these tips work with many other sites.