A parent called me recently for some help with this very good question. Like a good dooby, this parent followed all the advice given by parenting experts like me, "let your teen know that while you disapprove and don't want him/her to drink, you absolutely want them to call you when they are so you can pick them up, no questions asked, and get them home safely. Ok, so this is what this parent did, but now this teen has walked in the house seemingly with a free pass to drink, and have a "car service" pick him/her up to boot! Isn't this a mixed message, you might ask?
It absolutely is. Drinking/drugs and adolescence is not a black and white issue. If you say, "you are not allowed to drink!" Your teen will go underground, drink early in the evening, sleep over friends houses, chew tons of gum, or master the art of acting normal, or maybe not drink. You can always hope. And by the way, not all kids drink, and some kids will actually follow that rule. But honestly, if your teen is not a drinker, you would know, and wouldn't have to put that rule into place anyway. Many teens, thankfully do not want to drink. But for the many that do, you want them to be safe. The devil you know is better than the one you don't.
So back to the question. Now you have proof that your teen drinks, cause they asked you to come get them. But they don't exactly get off scott free. You have promised that there would be no direct consequences, ie grounding type punishment, but you still have the freedom to deal with it. You might have the following conversation: " You made a good decision last night, and for that I am really grateful. Obviously I am unhappy and disappointed that you drank, especially that you drank and were so compromised. You need to help me understand how that happened. And how in the future you can guarentee your safety. I get that the kids you hang out with like to party. That scares the sh** out of us. The fact is that you were sober enough at least to know not to drive and called us, but some other time you might not be so together."
And here is the best you can do the next time and every time thereafter they go out by saying: "Unless you can agree to stay sober tonight, we don't feel comfortable with you taking the car, and we will be happy to pick you up wherever at whatever time we agree on. Having the car gives you freedom but freedom and alcohol just don't go together. We love you and want you to be safe."
That is really the bottom line. I wish I could give you a magic answer that doesn't sound like doubletalk. Forbidding something you have no control over does no good. Punishing them until the cows come home, rarely has the long term affect you are looking for. Taking away the car or making yourself be a chauffeur may provide them with enough discomfort to not make drinking the priority of the evening. You will have to be the judge of whether your teen is getting trashed every weekend, in which case there is much more going on than just partying with friends. This kid has a problem that needs to be addressed in a serious way. If your teen is more in the normal range of a few beers or drinks but seems to have control, finding strategies that keep them safe is the goal.
There are no easy answers. Just keep the communication going!