Well, I'd probably talk to the girl myself and tell her that I know she's a freakin' plagiarizer, and that she better cut it out before she gets her butt expelled. As far as doing anything officially though, I probably wouldn't rat her out since these things can have serious rammifications and I don't really want to feel responsible for kicking someone out of school or whatever. I mean, it's unethical but I'd still feel kinda crappy for trashing someone's academic career.
Heh Plagiarism is one of those things that I just realized that I probably haven't done since like...1st grade when I used the encyclopedia for everything, but back then it kinda wasn't really Plagiarism cuz the teachers kinda told you to copy the encyclopedia lol.