She's 16, so not the end of the world to be starting at this age I suppose 
She moved school about a year ago, after being miserable, lonely and anxious at her old school. The change has been a lot better than expected. She now has made several good friends and the school-based anxiety has gone.
The kids in her class often have parties, which are pretty inclusive. She has been welcomed from the start. Even former classmates who have since moved to other schools come along!
When my daughter first started to go, she told me that the others drink. She told me it was her choice not to, as she didn't feel comfortable with it (particularly with her anxiety) and I obviously supported her in this.
Last night, her younger sister confided in me that she has started drinking at the parties. I don't suppose it's a massive shock, as she'll want to fit in, but I'm not sure where to go with this. She's a pain in the arse in some ways, as most 16 year old girls are sometimes, but has always been super sensible.
I don't want to ban her from the parties, as she is at last enjoying a good social life. I'm not even sure it's realistic to stop her from drinking completely. But I am worried and want her to have her head screwed on over this.
How should I handle it? She has apparently sent her younger sister (13) photos of her drinking and vaping, so that should be a hard no from the start.
I know I sound stupid and naive, but I wish my daughter felt she could talk to me about this. I'm not a total dragon.
Thanks.