My son was homeschooled for K-4 and then went to a very small private school for 6th and 8th grades (skipped 5th on entry and then skipped 7th). He was accelerated three years in math (so he did Algebra I the first year and geometry the second).
The math placement was correct, but the other classes, even with the second skip, were hit and miss in terms of an appropriate level of challenge--and the pace was always too slow. But the executive skills required, for him at least, bordered on too much. It's not that he couldn't do it, but I did have to orchestrate homework and planning a bit more that I'd like. We ended up homeschooling again after that 8th grade year.
It's been two years now, and he will be 9th grade age in the fall. He has decided to go to the public high school as a 9th grader. This way he can be with his group of friends from the public homeschool support program he attended in K-4. Most of his classes will be things he's had before, and he tells me he's ok with that (this is the same kid who would have a meltdown if the work in our homeschool was even the slightest bit too easy or repetitive). To the school's credit, they actually placed him appropriately in math without me having to do much in the way of advocacy.
A friend once told me that her philosophy was to place her (HG+) kids where they'd do best socially because the academic piece would always be a poor fit. When she told me that, I thought she was nuts, but now I see her point.
