It sounds like she is managing a lot - school, OT, social skills group. It’s probably worth dialing back your expectations around grades until she is in a better place mentally.
Identify what areas she is specifically struggling with and prioritize one or two things to work on.
My 17 year old AuADHD DD had similar experience this year, falling apart sort of all at once. I was worried she would refuse to go to school. We focused on switching medications and getting her therapist support for social anxiety. She has steadily improved which gave her the bandwidth to get grades back on track, have more energy to do things at home, etc.
It also may be helpful to give her something nice to look forward to like a special activity, knowing that she is struggling. It doesn’t have to be tied to a goal but a reward for putting in effort and working on overcoming her difficulties.
Hang in there, listen to her and be patient. She will get through this one step at a time.