Rent and watch the movie 'Mean Girls'. It's a hilarious caricature of female adolescents, but you can get an idea from that of what most of the media thinks of teen girls. (almost as bad as that, but less corny).
Generally, we're portrayed as promiscuous, impulsive, rebellious without reason, defiant for the hell of it, in love with dangerously wild parties, prone to lack of common sense, and with a tendency to either fulfill gender stereotypes or be the complete opposite of said stereotypes.
