…I think I might be relevant because I also have a 16-year-old at home (although I am a generation older than you…apparently I had kids late and you had them early).
I also started smoking at 15 but hadn’t smoked for 25 years until I started medical marijuana use 3 months ago. Due to that perspective, I will echo what others have said, that she might literally be self-medicating for some condition or other, and if so that condition might be of concern to you. I have experience with that kind of thing too, but I’ll leave that out unless you’re interested.
It’s always possible, of course, that she’s a Typical Teenager and simply “can’t live without it” because all her friends are doing it and she feels it’s necessary for social reasons. In that case, I would agree: she’s grounded 🙂 I would also be far more concerned about alcohol use, not just because alcohol is far worse for you physically than MJ but because alcohol use is more correlated to violence, rape, and auto accidents than any other drug except perhaps meth and PCP…and, unlike MJ, it’s probably available in almost every household.
I know the feeling about the lying…it’s the shock, then after that you worry about all the other things she might still be lying about. And you wonder what the hell is wrong with kids’ lives that they feel they need stuff like this.
Since you say she’s pretty awesome in general, and she’s proved that at least by her grades, I think you ought to talk with her, which you’re doing…and also listen to her. But still you gotta be the parent.