Everyone's incessantly telling you what to do at school. Parents and teachers are insisting on what you must learn, which students you should and should not go out with, what you can wear, and even what you should eat. It's a long list. You'd think you're in some kind of prison building or something like that. You would expect that since you're forced to attend school and all that stuff, they'd at least have the decency to let you browse whichever sites you'd like as long as you're there. But, no. The interwebs are censored, lots web sites are blocked. Even so, there are many ways of bypassing that at least, and it's a good deal easier than convincing your parents to homeschool you.
An unblocker is like a gateway that pass information between your computer and the site you're trying to visit. The unblocker accesses the site you want in your place, accesses the page, and sends it off to you. So the site you want to be on never sees your IP. So actually you are browsing websites in an indirect manner. School filters only see you accessing 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 if they block it, you can make another. How to do this is too much effort to explain here, if you do a search for "How to make your own proxy" you will find info. You can also make money from this through advertising.
Other ways to get on Facebook are by using translator sites, like Google Translate or Babelfish. You can also try using Google's cache version of the site. Unfortunately, you can't to login that way, so it's still better to use an unblocker for Facebook, though these tips work with most websites.