Too cool for school

This describes someone who is constantly judging everyone and everything to be less cool than they are, because in their mind they're a true rebel in the know about everything. This kind of person mocks or rolls their eyes at anyone who they judge to be uncool, but only because they imagine that other "cool" people would hold such a view, and they feel they're in the know about how all the "cool" people think, and want to feel accepted by the cool crowd. They're simultaneously insecure and worried about maintaining their image at all times, and in case anyone might see them, they won't let themselves get really into or become enthusiastic about anything, lest that thing be thought of as uncool. Also applies to hipsters on the excessively surly and douchey end of the hipster spectrum.