Where can you find a chat room that your school isn't able to block?

The cafeteria would work, especially if you sit close enough that others can't hear what you're saying. You could try the library too, but you'd have to speak in whispers. Maybe homeroom?
On the other hand, if you're talking about INTERNET chat rooms, maybe you should just get your own internet connection. If you don't want to do that, you'll just have to put up with the school's attempts to instill a modicum of morality... even though most of you don't even understand the meaning of the word.