It's an american public school thing (so I've heard). Cool girls (in school) are girls who think they're better than everyone else, are very popular, have a lot of social power, and look down on pretty much everyone. Cool girls come in packs, like wolves, and verbally (if not physically) attack other girls (and sometimes guys, but mostly girls). These girls have no pity and often turn on each other when doubts arise. Usually, there's a cool girl ring leader, and the other cool girls are her fearful disciples; striving to be popular, while terrified that the ring leader will abandon them on a whim. These girls will do anything to meet her approval from turning on each other to torturing some innocent weirdo/nerd. 
