I understand your grave concern, especially in light of the ED. I, too, was truant and smoked pot in HS as well as into college, until I realized how ridiculous I was when I was high... dissolving into hysterical laughter over the silliest thing for about 10 minutes, eating everything in sight for the next 15, then falling asleep. Whoo! Are we having fun yet?
Your kids have several huge advantages: They're well-educated, have a vigilant mom who cares about them tremendously, and know better (they really do). I would venture a strong guess that they are just experimenting, and your getting through to them will hinge on your ability to be very calm and matter-of-fact. Which, it needs not be said, we all know you've got that!