I know this will happen to me soon enough and i can imagine how tough it must be.
I will not permit my daughter to drink but that does not mean she wont. My dd just yesterday showed me a pic of one of her friends on my space posing with a bottle of alcohol. She said look mum she is only 13 thats terrible she looks stupid and thinks she is cool. I then made reference to the fact that drinking doesnt make you cool etc.
Im not sure what kind of punishment you can give her, the main punishment that works around here is no mobile and no internet. Not sure that is appropriate for this situation but i dont know your dd, i think it depends on the child so maybe Lulu is right ask her what punishment she should recieve.
Sorry i am not much help, you are doing a great job and i am sure you will make the right decision and i hope your daughter makes the right choices in the future.