I share your anxiety, because teenage drinking today does not seem to be 'a sip' or even sharing a bottle of Bulmer's, which was the position a few years back. The kids are able to access very potent drinks and it is 'cool' to be seen consuming them. What was tiddly and giggly once is now quite frequently seriously drunk and incapable. Then the consequences of this kick in, and this is more dangerous for kids - as a parent, try going on a social networking website used by kids such as Piczo or Bebo and just see the material posted.
A couple of thoughts: you say your daughter has been doing well at school. Is she seeking dodgy friends so as not to be seen as a nerd - the namecalling in schools will include Ugly Betty and a host of hurtful names and phrases aimed at girls who succeed. She may need supporting in finding confidence to be able to mix with people but not drink - to have confidence in getting out of social situations as well as falling in with them.
Also, is there any mileage in having social events - controlled, with focus and limits - at your own house, so you have control and overview of what's going on. eg a clothes swop, where girls bring their unwanted clothes / shoes / handbags, play dress up and agree on exchanges.
It's a scary time, but hopefully you'll both come through. My daughter became human again at seveteen, and no lasting damage. Good luck.