I'm pretty sure my highschool uses a word filter thing for most of it's censors, which is pretty effective, however they do have to go in and manually block sites that slip through the cracks, or unblock useful sites that contain the blocked words. Youtube was blocked up until this year, now it's just restricted, and uses it's own special filters to block videos, not sure how this filter works. Facebook and Myspace are blocked as well.
Also, for most censors like this, if you want to get around them(without using a proxy), just put an "s" after the "http" in the url. Of course, you have to do this for every individual page, so it can get a little tedious if you plan on browsing around.