Stealth, with due respect you have a 6 and a 3 yo so have no experience of teens, alcohol and parties apart from your own. I have an 18yo and a 16yo.
My teens have been going to parties for 4 years, DS1 now goes to pubs and clubs too. We have a very responsible attitude to drinking in our house. I have never been drunk, ever, not even when I was a teen. I am teetotal, DH drinks minimal amounts, maybe a bottle of beer or small cider on a Saturday. My DC have never, ever seen DH drunk (and neither have I). We have educated our DC about drinking responsibly, alcohol has never been the forbidden fruit, they were allowed wine with lemonade in at Christmas from about 10.
When they started to be invited to parties, we knew they would be responsible. They knew they were not to come home drunk, or end up in A&E. They had no interest in getting that drunk. DS1 said the other day that when he is on holiday next week he isn't going drinking to excess every night, he is going to go to the old town with another boy for a couple of nice meals.
DS2 went to a party on the day he left school last month. A boy there was allowed lager for the first time. Within 2 hours he had drunk so much he was vomiting copiously and an ambulance had to be called. That is not the first time that has happened in my boys' circles of friends.
Just because you can not find any 'evidence' doesn't mean it doesn't happen.