I've spoken with both my High-School age daughters:  If they choose to try or use alcohol or cannabis only do so in moderation, only among trusted friends, and definitely don't succumb to peer-pressure from any idiot seeking to get them wasted.  

For under 18, cannabis and alcohol are both illegal.  Personally, I think my daughters would be smarter to try a puff of cannabis over getting drunk.  Cannabis is safer choice over alcohol.  

Of course it's best if they don't do either, but that's just head in the sand thinking.   It's not going to happen.  Teens want and need to experiment a bit, and inevitably they're going to give weed and alcohol a try.  

Having great role-models, keeping lines of communication open with the likes of yours truly, having great and attainable goals in their lives, and witnessing a family lifestyle of moderation is the key to successfully navigating their way through teenage wasteland.

So far, so good.