the immediate concern, I feel, is to get her through her exams as best and stress free as you both can and leave this unhelpful school behind. Complaints can take months , even years (I know ) You can fight the school later but maybe keep her out of it , as in the short term, they may make things harder for her....
It sounds like a horrible atmosphere for her . If she is' at risk of permanent exclusion'(which it sounds like she would be if she was staying), she might come under jurisdiction of the Vulnerable children team at the LEA. Try them in case they can fund a home tutor to help with revision (if your DD wants revision support)
The Head of the Pupil support service at our LEA really helped my DD back into education, so maybe try him/her directly if you can prove to them she has not been given adequate teaching for the exams. get a letter from camhs saying how important her education is to her mental health, and how denying it has risks and give to the LEA.(more so than governors at this stage I wonder - governors may protect the school, but the LEA has the responsibility to educate).
My feeling from experience is focus your energy on you, her state of mind and a brighter, different future and have as little contact with school as possible, revise whatever she can under no pressure, sit the exams, and then, as you say, look at the future with hopefully camhs support and a new environment. If necessary she can retake elsewhere next year, my DD did and more do than you imagine. The important thing is to get her the right support to cope at school/college and start believing in herself again - and coping.
Let her know that her happiness and her health, not the exams are the most important thing at the moment, and college/schoolwork/exams can all become more important again when she is feeling stronger. Help her with relaxation and try to stay calm around her (its so hard I know ).
best of luck to both of you........