It's so hard for them and isolating if they don't join in with their friends. Let's be fair it's tough for them to gracefully decline without feeling they are lame or losing face. Does anyone have any good responses for kids to use if offered drinks by their peers?
I think all this comes from how you bring them up from when they are small - working on their self esteem, encouraging them to try things thier peers might not do. Helping them with how to respond if people tease or mock them for any of their choices (be that fashion or the activity or hobby they do or the bands they follow or whatever).
Also important to talk about situation when they don't directly impact on them - either when they come up in some TV or on a film you are watching, or something that has happened in the news, or you can bring into conversation a 'situation' that you heard about that happened to a friend, or colleague's dc (fictional or not).
It is much harder if this is the first time they are making decisions they think might not be 'popular' with their peer group.
Also, I am trying to avoid giving lectures as I was the same age when I started having the odd drink, but I think it's too young at just 15.
You can give information, without it being a lecture. Information about the amount of alcohol in different types of drinks. Information about eating first (or alongside). Information about drinking water in between drinks, and the need to stay hydrated. Information about the dangers of mixing drinks.
Also, having modeled a good relationship with alcohol over the years. Whenever we have had drinks as adults, we've shown that it is fine to enjoy a drink, but that it doesn't have to be a race to get plastered. Also that it is equally fine to not have alcohol and to still enjoy the party / occasion.
Then - let them know that you will ALWAYS come and fetch them if they feel uncomfortable in a situation. Use a code word if you must... maybe if they mention the (fictional) family dog in a text, you know it is time to ring them and say there's been a family crisis and you need them to come home immediately if that is what it takes.
All that said - 15 is too young to be taking alcohol to house parties, IMO. You are the parent. Let them blame you, to their friends if that helps. One thing having a taste / small glass with you as a family on occasion, but a different thing encouraging them to drink out with their friends.