It is a shame when it comes to this, but it will be better for your child. I have seen first hand how much difference the right school, the right fit for one unique child, can make. There is no reason for a child to ever fail to thrive in a school, because it seems to me that with the right placement every child WILL thrive.
I know it will be a difficult decision, and the adjustment will take some time, but I really hope that what you will see is a happier child who begins to enjoy, rather than dread, school.
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Mom to an amazing young adult AS son, plus an also amazing non-AS daughter. Most likely part of the "Broader Autism Phenotype" (some traits).