@RonObvious
My DD dropped out in the first year of a levels
At the time she had no diagnoses but was clearly anxious.
She was also very physically unwell with significant joint pain and fatigue.
She was referred to the me/CFS people and was eventually diagnosed with CFS, fibromyalgia, hashimoto's thyroiditis, adhd, anxiety, depression, and is now in the process of getting the autism diagnosis as well. (I mean technically they are assessing her because they wouldn't assess her until she'd been on the adhd drugs for two years so they could see "what was left" and it turns out there is a lot left).
Because of this we know quite a lot of people who have dropped out of school at various points. There are alternatives to mainstream school and my DD did much better once she felt in control of deciding what courses she did and her general academic direction,
College was much much better than school (she dropped out of a sixth form where she had had a scholarship). Some colleges offer a 14-16 pathway for students who have been home edded/sick etc to get GCSEs in a non school environment and the students I know who have done these found them much better.
Btecs and access courses are also very good for applying to uni from - my DD did an access to science course which she topped up with a level maths studying herself and sitting the exam as a private candidate.
The access course was at an FE college and they were very supportive but also the lack of school pressure (you must revise 2 hours a night or you will fail kind of thing) made a massive difference.
I have worked in education mostly as a teacher for twenty years and there are plenty of kids who really just don't cope well with the pressure and social expectations of school. It doesn't mean they won't cope with adulthood, just they need a different kind of support to get there.