This issue popped up on the well trained mind forum recently. Basically the kids can graduate early by testing out of high school. I am in another country so I don't know the names of the test but basically from 13 they can sit a test to get a high school diploma and graduate. This fees them from high school. In most states any study is enough to satisfy the criteria. So you might homeschool high school while being fully enrolled in college. Homeschooling high school could be exactly the same units as what they just happen to be learning in college.
if it needed to be.
I think every situation is different and it will require some digging and phone calls to find the right loopholes. Boy did I spend some time looking for a way into college early for my kids! Now I am on the hunt to make it affordable. The thing with putting kids into college early is that they are accumulating dept before they are old enough to understand what that means (at least in Australia they do) so scholarships become all the more important. Skipping high school does give them an extra couple of years of earning capacity though!
Ring a few colleges, ring the homeschool legal defence, ask at a few conventions. Your answer is there. Keep digging.