I am so sorry you and your dd are going through this.
Agree with cauliflower big time.
Also good advice from matimeo and PoppySteller and all the others giving excellent advice. Support school where you can but when what they want to do is counter productive, Eg reintegration when nothing has been resolved, then say what you feel and get support from Gp and post adoption support to exploan why this is not in her best interests. It must be very hard for teacher and rest of class too.
It almost sounds Iike she is trying to burn her bridges, alienate herself from rest of class.
Does she have friends? Have you met them with her or spoken to the mums/dad (primary carers) to find out if anything has happened.
It is a bit confusing whether this behaviour is totally out of the blue because your recent update said she hates school and I wonder if this has been simmering below the surface for ages?
If genuinely out of blue I would be thinking bullying, (sorry) some sort of physical or even sexual assault that has happened and made her not want to leave your side, out of the blue anxiety attack which has made her feel school is not a safe place and she must escape at all costs, etc.
I developed anxiety in my early thirties. I had cognitive behavioural therapy for it(on the NHS) and am totally fine now. It did not make me behave like your dd, but then I was an adult. (What I am saying about me is true but as far as your dd is concerned this is total guess work, so do feel free to ignore me.)
If this has been simmering a while I might think mental health issues (do birth parents have them?)
Could this be as yet unrecognized or undiagnosed Attention Deficit, ADHD (more common in boys but possible in girls I believe),.ASD, PDA, ODD. Our birth dd had none of these extreme behaviours but some others at home and we considered all these different conditions for her with CAMHS.
Just so you know, I'm mum to a boy of 6 who was adopted at 3 and 3/4 and is in year 2 working at year 2 level, and having Theraplay for his emotions.
As I say, I am also mum to a birth dd with autistic tendencies at high school working several years below her age.
I think that you need specialist therapitoc advice, I wonder if therapy or something called theraplay could help? This could be paid for by school using PP money if they agree, by post adoption support (even if they have no one to provide it, if they have a fund to fund it you MIGHT be able to access that). You can pay yourself, if you can afford it. If you do, save all receipts, unlikely you will be able to claim back but who knows. Also, if you have any kind of private family health cover then you may be able to use that.
Lastly, and I am sure you know this, ctovitoes side by side, play dough; craft; baking;.decorating cakes; gardening etc will give good chances for her to open up. She says she does not know why she is doing it but she may be able to leak out clues as to what is going on internally.
Please IGNORE anything I have said that is unhelpful. 