I'm sorry I can't address your questions about HS, but I have a 8yo 4th grader. It sounds like your DD's situation is very much like my DD's was.
My DD has an early September birthday and was early-entranced to K just before she turned 5. In 2nd grade she began going to 3rd grade all morning for math, reading, and writing (and art and P.E. because of the schedule). By the end of that year we knew she needed to just skip to 4th full-time this year. For one thing, she was doing all of the academic stuff in 3rd grade, not second, so she could definitely do well academically. Also, elementary curricula tend to try to interrelate all of their subjects (e.g., they study math during science and vice versa). Third, it is much less obvious to the other students and therefore less socially stigmatizing for her to be in one class all the time rather than constantly coming and going.
Anyway, again not HS experience, but our experience with the type of grade acceleration you're talking about, is that it has been really wonderful. She is still at the top of her class (not always *the* top, but always one of the top)and she has made friends really well. In truth, no one who doesn't know the situation has ever picked her out from among her classmates as different. There are kids 1.5-2+ years older than her, and yet she fits in better than she ever has and no longer has to downplay her abilities.
Everybody always talks about things like puberty being of some concern, but honestly, kids go through puberty throughout a large window of time, and who's to say that your child won't be an early bloomer or wouldn't have been a late bloomer anyway. Also, the driving thing comes up a lot and I think that nowadays driving isn't quite the universal 16-year-old milestone that it has traditionally been. Still, though, even if these issues come up, I'm prepared to talk to my DD about them and console her if she's disappointed. Either way, I think any short-term issue that comes up will be easier to deal with than the long-term ramifications that could come up if we didn't address her academic needs for years on end. I hope this helps, FWIW!
