If you feel that a group should abstain from a particular activity for the simple reason that they lack the ability to fully appreciate the consequences of carrying out such an activity, then what sense does if make to try to convince them of that? Indeed, either such an enterprise would undermine the very basis for having them abstain from the activity in the first place (by helping see the possible consequences of a given course of action), or it would be a complete waste of time (i.e., they would not grasp the link between a given course of action and a possible outcome). However, such seems to be the case for many school programs. Teachers regularly delineate possible outcomes of certain activities (e.g., choosing to become sexually active). They then test them to see whether they understand these links. At the end of the day, however, no teacher that I know tells students that have mastered the subject matter that they should now feel free to become, say, sexually active.
At best, what can be said in the case of alcohol is this: "Yes, there are plenty of teenagers that know how to drink responsibly and you might be one of them. However there is a critical mass of teenagers that do not. With this in mind, the courts have set the drinking limit at 19. Now, in order for the law to be workable, the law must target all of those under the age of 19 and not just those who drink irresponsibly.
Continuing on in rant mode, it is clear that the just say no drugs and alcohol model simple does not work. Now, let me add to the speculation as to why. Somehow it is not dawned on the just say no crowd that some teenagers will continue to drink and do drugs no matter what and that by tailoring their message only to those kids who are having drugs pushed on them they are, among other things, failing to reach one of the most influential segments of teenager culture, viz., those that push drugs onto other kids. One needs to acknowledge this group and teach them to respect those who refuse their overtures.