We have various filters in place. These filters are designed to achieve various goals.
One is to prevent bandwidth-hogging (we can't afford a gigabit fiber run to the Internet backbone, so we have to share our bandwidth wisely). Nor do we feel compelled to to pay for content that hinders the academic process (see below).
Another is to prevent "time wasters". How many schools let kids bring in their XBoxes to set up and play during class time? They are there to learn, not play games, socialize on Facebook, etc. I find it funny how many will rant about the situation of American schools vs. others, especially in math and science, and then go an suggest that kids be allowed to do whatever they want on their laptops during class. (BTW, our filters switch into a "relaxed" mode at the end of the day when kids are in study halls with little to do.)
Another is to protect them from things like online pornography, etc. I'm not even going to waste time as to argue why this is a good thing.
Another is to protect the network and their own computers from spyware, viruses, etc.. Our network is proactive in that it will cut off any computer that aggressively tries to "break out" or behaves like it's infected.
Since filters are not perfect, a report is generated weekly for each teacher, showing them exactly what sites their own students are visiting and during which classes. Technology can assist good classroom management, but it can never replace it.