Kathie's right, school's use filtering software that is loaded onto the server so the network people need to get involved. This is a big deal and there's lots of informationn out there as to what other schools are using. (Some things can be set at the operating system and browser level, too.) The technicians can configure the software in a variety of ways. For instance, ours blocks out most things of a sexual or gambling nature. It also leaves an audit trail.
Of course, sometimes legitimate things get blocked, too. We can't download the application for charitable bingo (for Spring Fling) at school because it's on the Texas Lottery Commission's site. And "lottery" is one of the blocked phrases, so the computer won't go there.
I know this is a serious issue, but sometimes there a funny parts, too. This year we had two boys try to access some adult sites and got caught. (Someone had given them some addresses to try, but the filter stopped them.) When the principal review the computer logs, she saw they had tried to go to a particular site and then to a fishing site (which came up). She couldn't understand why they were going back and forth into a site about FISH when they were obviously looking for adult sites.
Turns out that as well as lacking judgement, the boys could also use a spelling refresher. There were going to:
www.stripersonline.com/
Then again, maybe it's good that 9 year olds don't know how to spell "strippers".