This exact thing happened to DD2. Also autistic, diagnosed early year 7 due to waiting lists.
She started with emotionally based school avoidance after the Christmas holidays of year 10, was put in R&R (small base), then didn't cope so was given a reduced timetable, then didn't cope, and eventually by mid February they told me to stop trying to get her to school because she was so obviously distressed.
I applied for an EHC Needs Assessment, then as that process was underway, she was offered a place at the local PRU in their 'vulnerable students' centre. It was part time with just 2 or 3 pupils per session.
Eventually she got her needs assessment and I requested a sensory OT assessment and a Speech and Language assessment as part of that. A bit of a battle, but we got them.
After all that, she was given a place in an independent specialist school. She has 3 children in her class, with a TA who accompanies them to all their lessons and is 'their person'.
The school is brilliant. DD2 went on her practice D of E walk this week. She was in a terrible state and we had to put a lot of things in place for her to attempt it. In the end, predictably, she didn't use most of the measures but for her, knowing they were available was what enabled her to push through.
The difference in her is remarkable. She actually talks at this school. She has weekly SALT sessions to work on her mental state and communication skills (although her language is ok, she finds it very hard to express herself and gets too anxious). She also has weekly OT sessions to work on her sensory needs. Her school has a farm, so she spends one morning per week at the farm, which she loves.
As well as studying her GCSEs in English, Maths, Spanish and Science, she's doing Functional Skills in both Maths and English, which takes the pressure off her GCSEs, and she's doing a vocational qualification in catering, a BTEC in IT, and a BTEC in construction.
DD2 is still struggling with anxiety. We're still on the CAMHS psychiatry waiting list. It isn't easy and she's not managed to be full time in school yet. There's a lot of damage to be undone from her last school. However, she's doing really well and she is improving.