I would not send my kid to wilderness school before exploring meds -- it can take time to work through different types -- anti-anxiety, anti-depressant or mood stabilizer. Are you working with a psychiatrist in addition to a therapist?
Re: school. School is not working in COVID for a lot of kids. If your kid is in middle school, I would not worry too much about grades right now, mental health is the priority. Has the child been assessed by a neuropsychologist? There may be underlying learning issues that are increasing anxiety, especially in the DL format.
Is the anxiety becoming a form of school refusal? I happened to notice this place when researching something else --
https://changeanxiety.com/services-specializations/anxiety-based-school-refusal/ It is in Rockville.
Do you have a 504 plan or IEP for your DC who is experiencing anxiety? A 504 plan could offer your DC extended opportunities to cut or make up work when missed due to anxiety or depression. Depending on how anxiety affects school, there may be other accommodations available.
I would hope that all of the above have been used before sending off to boarding school.
FWIW, my DC was extremely oppositional. I thought it was anxiety or LD frustration, but it was actually depression and medication made a tremendous difference.
I agree with other PPs that "wilderness schools" are to be looked at with extreme caution as many of them have been found to have abusive aspects or practices. Force is not the antidote to anxiety.