My son has just graduated from high school at age 17. He has ADHD and anxiety, and while he was always intellectually capable of doing the work at a higher grade level, socially and emotionally he struggled. I started discussing having him repeat a grade as early as kindergarten. When I went to talk to his teacher about it, she said “You can’t hold him back — he already knows everything we teach in kindergarten!”
I had the same conversation at various points with other teachers and counselors over the years, up through middle school. I found, though, that each year they became more resistant to the idea (I think because of the social stigma a kid who repeats a grade will suffer).
School was a huge struggle — he was smart enough to take all advanced classes, but emotionally and socially there was a lot of stress about keeping up with assignments. We struggled to find the balance between helping him meet his academic potential while also getting him the social/emotional support system he needed to get his work done. It’s a tricky balance — and one that schools don’t usually have a system to address. If my son had had another year to mature, though, there would have been a lot less stress about getting schoolwork done, etc.. If I had a time machine, though, I would go back and fight the teachers and have him repeat the grade.
That’s my 2 cents. Good luck!