TTNET (biggest ISP in Turkey, founded and formerly owned by the government) gives you username for your DSL line, its usually something like phonenumber@ttnet (or yourname@ttnet1). If your username is like that, you connect with TTNET's DNS servers and your only limit is the sites that are blocked by courts.
But, if you're a school, they'll give you* a username like "schoolname@meb", when you connect with this there is a special black list (which includes word filters too) that can't be passed with anything but proxies. And, BTW, "proxy" is a blocked word.
For example, you can't -even- search for "porn" on Google. I'm not saying you'll need to search for porn on Google in school, just explaining how this black list works.
* A friend of mine explained it to me like this, he was a computer teacher in a government school. Now the system may be changed, I don't have clue, but the block is still there today.