Welcome @IAmNotALoon, what a lot of strain you are under. No sight of hijacking here.
What is her current plan for 16+? Does she have a plan B? Are they threatening that she'll miss prom too and if so, does she care about that?
You can have a phone appt or do an e-consult for her and share your concerns with her GP. They won't be able to tell you much in response because of confidentiality but it's still worth doing.
It sounds like a complicated mix of understanding and not understanding from school. Maybe try to have a meeting with them about getting her back into school. It might end up centring round the phones but it'll go down better if you frame it round getting her in. There might be accommodations or other forms of support they can put in. I doubt they would sanction her using her phone to support others' MH when she should be learning - that sounds like it would not be safeguarding her - but a conversation might open up a route to some other ways forward . It's developmentally normal at this age to be quite black and white and think there is only one possible solution to problems but there may be some "third ways". If nothing else it sounds Ike a heavily reduced timetable might help, if she had not already got that. They may not be terribly helpful now it's been framed as a discipline/suspension thing, but on something this important it has got to be worth a try. Maybe you already have though and I'm just stating the obvious
Absolutely no one can make her sit her exams, true, but we were told fear of exams is often rooted in fear of the yawning chasm of change and uncertainty that follows it. That chasm is even bigger and scarier if they don't sit their exams so when it comes to it she may be able to give them a bash. If you hate maths and English, being able to give them up is a big prize.
I really hope school will be used to school "refusal" this close to exams and will work with you.
@Leafywool thinking of you and DD today x