How much of the friendship is based on pressure from the front-runner friend who wants to be 21 today?
If that sort of pressure persists and the friendship is based on trying 'new stuff' and being 'bad/naughty' I'd question if it's the right place for my DD.
You can't ask her to say no when it's impossible not to affect the friendship? Already you're having to keep schtum because the friend set up your rules too now. How far does her influence reach?
That said I wouldn't tell the school either.
Can she branch out and meet new people ,new friends, new hobbies? I'd be looking to try and lessen her reliance on this friend and also teach her about what friendship really is.
yes it's about trying new thing together - but not weed at 12. That's a too young. And for some they'd say it's never the right age.
Remember that weed affects the teenage brain and can bring on all sorts of anxiety disorders.
The self-harming is also a worry. She's a follower not a leader. She'll follow the pack, she's easily influenced by the sound of things and wants to belong in a group. Perhaps you have to start to wise her up to the fact that in life there are choices, good choices and bad choices and people will push you one way or the toher but in the end, you have to decide for yourself what is right or wrong. You may not be liked for that decision - but is being liked more important than your health? Your safety? the law?
Does she want to be a follower, or a leader?
No good shouting at her or threatening her. Educate her. Properly. Show her some documentatries on the usage of cannabis. Find some medical studies about the increase in pscyhotic episodes. Don't freak her out completely but show her the truth.
maybe show her county lines, how it quickly turns into something sour and grim.
For me personally I'd be looking at changing the school and to a much better setting.