well, something similar happened to me when I was a teen. i wasn't driving, a friend was. we were all underage. got pulled over, and we had beer in the car....so, the cops gave us all breathalyzers. none of us were drunk at all, we all had a very low amount of alcohol in us, not even close to being above the legal limit. so, the cops said we basically couldn't drive the car, and took us to the station, took our beer. that was pretty much it, no underage drinking charges or anything. they even gave us a ride home...lol.
i would say we got off pretty lucky, but we were also very cooperative, if we hadn't been, i think we would have gotten off worse.