Whatever your views on censorship of the Internet, I think it can be argued that preventing phishing at least is a good reason for some filters. Our school district uses FortiGuard which is administered at the Central Office, but for the network in my classroom as well as at home, I use OpenDNS.
OpenDNS can be set up on each computer, or in the router that they all use to connect to the Internet. You can choose categories of sites to block, or even blacklist or whitelist individual sites. It's quick and easy to set up and make changes, and it's free.