Hey OP,
Sorry to hear your daughter is feeling so low. Honestly, I really don't think Mumsnet is the best source of advice for you with this. I would seek information and guidance from those with specialist understanding of autism. As she is shutting down communication it must be hard to understand what's really going on for her and therefore the best course of action. A therapist or counsellor who is ND-informed might be able to help with this.
I have a sense that there is a core issue of knowing she is 'different' but constantly trying to mask it because she sees difference as a categorically bad thing. So I'd be looking for opportunities to shift this mindset and grow self-worth and pride in her individuality. Advocates for the autistic community who have podcasts/vlogs? ND people working in skincare or with cats who she could look towards as positive role models?!
It could be worth finding home education networks in your area who will be able to point you towards alternative learning communities. Often these run 3 days a week and offer the social connection and shared learning opportunities that people worry about missing when leaving school. Maybe visiting something like this with your daughter would open her eyes to another possibility, rather than feeling trapped in a conventional secondary. These huge settings are overwhelming for the senses and the social norms can emotionally trash people who don't 'fit in'. In a different kind of educational community they can regulate their nervous systems, be celebrated in their uniqueness and positively thrive.
You could also check out Not Fine in School and Square Peg
Unless you are worried she is in immediate danger, I would try to work out a way of making this decision together rather than unilaterally. That might involve taking it slow and gently gently, because it sounds like she is in rigid shutdown mode, which is a survival mechanism. Feeling safe enough to see you as her ally and let you through the castle gates is a big deal (and even if she wants to rationally, her subconscious will call the shots). That doesn't mean no boundaries from you, but I think the ability to stay calm, patient, compassionate and firm is important. If you can get some support to resource yourself then you will be best able to co-regulate her nervous system with yours.
Hope that helps. Wishing you luck and strength in navigating this with your daughter x