Unfortunately I do not have useful advice to input here.
Had a similar situation with my own sister when she went away for college (The thing I was advising her not to do was probably about 100 times worse than this). Anyway, that situation taught me a lot about people in general.
But anyway, it's useful to think about it like this:
I was not the only person advising my sister on things, she has friends, she has sorority sisters, she has a boyfriend, etc. she has XYZ people in her ear influencing her to do certain things and she's young so of course she's going to continue doing it until she learns her lesson. Unfortunately for my sister, those decisions have life-long consequences for her that will never go away and she cannot take back.
And when working with teens/kids, you really just the same thing all the time: They are too young to make conscious decisions and to grasp long-term consequences of XYZ until they learn their lesson of course.
Anyway, I'm sure you know by now: the more you try to force it, the more they resist against you. Trying to brute-force anyone into stopping anything even if you are using logic, is a sure-fire way to have them want to resist against you even more.
If it was me, I suppose I would try to get her off THC and probably stick to Nicotine or Tobacco (Not ideal, but its better than THC).
If that doesn't work, then what can you do really? Even if you took it away from her you have to keep in mind she has friends, a whole school full of people she knows (probably the people who introduced it to her in the first place), that can help her just get another one.
Honestly, I wouldn't be too down on yourself if that was the worst thing she was doing, because I have seen so much worse working in schools and dealing with my own sister. I would just be mindful that continuing to do things like this leads her to a path where she continues to make bad decisions. So, I'm not sure - maybe try to convince her to switch off THC for something safer or make some kind of compromise. But anyway, like I said, I don't have any useful advice for this situation. Sometimes, kids just have to learn from experience.