ctmomof3 - I am so, so sorry for what you're going through. I have too many friends who have tried to deal with teen immaturity:
"The pot's only recreational Mom, I can quit anytime."
"Don't worry, I'll study when I get to college."
"But s/he really loves me."
"My relationship with him/her is WAY more important than a few classes."
"You mean they can fail me for missing a few classes?"
"I'll do better next semester."
What can I say that will assuage your concerns? Nothing, probably. IMHO you need to decide whether (A) you're willing to give your S the opportunity to succeed/failr, or (B) you insist on some commitment to success before you commit family resources to this endeavor. My prayers are with you.