I know some detail about two-- there wasn't one close to US either, so we were looking at a major move in order to make it happen, and well, that didn't happen.
I'm WAY less than impressed with a lot of the quality of distance courses. It's fine for credit recovery, but they are (even at the post-secondary level) mostly hit-and-miss at about a 50-50 rate when it comes to being engaging and authentically thoughtful.
I think that a lot depends on your student's particular learning style. Autodidacts seem to do okay with online courses. But recognize that it will definitely be missing the live instruction/interaction component-- no discussions in real time, for example. For some students, that's fine, but for others, it's a big deal. (My dd is one of the latter, let me say that up front-- her ideal learning setting is Socratic.)
Do you have a local college or community college? Many of those institutions are willing to allow high school students to take credit coursework quite early. My DD was taking community college (non-credit) classes at 10 and 11, for example-- though they DO want a parent to be either in the room or in the building, we've found.
Summer session classes would be another good option, though for HG+ kids, I think that the issue is to make high school more challenging and meaningful via differentiation.
We've used the following strategies there:
a) acceleration-- 3y worth
b) AP coursework-- DD has really enjoyed her AP classes, most of which are taught by the best/brightest TEACHERS. That's my insider tip about that.
c) dual enrollment-- but just WHAT this is varies tremendously-- in my DD's case, it is just classes that her high school teachers teach that have the "approval" of the local post-secondary institution
d) many extracurriculars (this is important come college-time, anyway)
e) summer enrichment (internships, etc.)
Some of the early college programs which are residential might be a possibility for some high schoolers. I know that one of the longest running of those is in Seattle at UW. They recruit 14-15yo into that program, and have for decades.
There is one modeled after it in TX, as well. (I know that there are others, it's just that those are the two that I know about.) The students in those programs are housed TOGETHER; they are really supervised quite differently than other college students on the campus, being so much underage.
