I can only (as others have done) share how
we managed this kind of situation with our EG/PG child. She is 11 and while the state considers her an 8th grader (that is, she has not yet been assigned a graduation cohort year), she is taking all high school coursework this year, and has already taken high school courses since she was nine, starting with algebra.
We've avoided the obvious chronological age difference issues by choosing a virtual charter school. This gives my DD (now 11) the freedom to choose
when her classmates find out how much younger she is. Mostly, they take it very matter-of-factly-- but then again, they've almost always had a chance to just get to know her as "that smart, funny girl"
first.We have opted
not to do a full "skip" since school entry (3rd grade placement at 6yo mid-year entry). Instead, we've compacted/telescoped curriculum so that she could smoothly move THROUGH the curriculum, which avoids obvious gaps. This has the additional advantage of allowing the PG child to work at the increased
pace that they often prefer. In a brick-and-mortar school, this can be done by placing the child in a split level classroom during the 'double' year, and letting the child work more-or-less independently through BOTH years during that time, thus preserving social relationships in the acceleration year and the one following. Peers will already "know" the student since s/he was in their class the previous year, socially smoothing things a bit. On the other hand, once you reach the 'destination' grade, keeping them in sync with that reduced rate of instruction again can be a bit of a problem, as we've learned.
The full picture of how we did things with DD:
We HSed for a year beginning when DD would have entered kindy as a young 5yo; we knew she was moving through scope and sequence at a tremendous rate, so we tested her (informally) using a 2nd grade CAT5 after about ten months, which she easily scored 99%+ on in all areas (her first experience with any kind of assessment at all). We then applied to enter the charter school when she expressed interest in "real school" in January of that year. DD did all of 3rd grade in about 8 weeks, then we filled the rest of the school year with electives (Calvert curriculum's elementary mythology, some Latin, and World history), all of which she finished as well. The following year, she finished 4th grade shortly after winter break, but then we had difficulty getting the national organization to ship out the 5th grade materials (yes, due to her age... <sigh>), which meant that she had nothing at all to do for ~8 weeks while we waited, and in the interim, she had a medical crisis that resulted in only getting about 6 weeks with the 5th grade material. So she finished the year only half-way through it. We (meaning parents+school) agreed that having her start 6th in the fall was still the correct thing to do, by any measure.
6th went very well, though she chafed a bit at the pace. 7th, she was less happy with the pacing, and we had a lot of trouble getting her to do assignments from mid-year on. Last year, the problems were pretty severe from January through late April... but at least she was able to happily plan for high school courses.
This year, we are realizing that the underlying
pace of the curriculum, even in "honors" coursework, is just too slow, and it has been since 6th grade; not only that, but the spiraling pedagogical underpinnings are a huge problem. She is really struggling with existential depression and is completely disengaged from school in many ways. Ordinarily, this is probably where we'd opt to accelerate again, but for reasons individual to DD's medical situation, we really don't want her graduating any earlier than 14. We're HOPING that we can do the next couple of years with her with a mixture of enrichment, core requirements viewed as "nuisance" chores on a checklist, and independent study. At that point, we'll begin to transition her to college coursework.
We are fortunate in that we have the opportunity to do non-enrolled community college courses at pretty much any point, and there will also be a chance to transition VERY smoothly from the community college into the larger state land-grand university with complete reciprocity at some point in the future, as well. Those
are things to look down the road at with PG kids-- just as long as we bear in mind that this is merely "plan A." LOL. We plan, but DD's a wild card and often as not, a complete game-changer.
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The good points are obvious, I think. Super-modified acceleration

(as opposed to merely "radical") has allowed her to stay engaged with regular school FAR longer than would otherwise have been possible, and it has been useful to have her experience the school environment (as opposed to completely homeschooling). She has made friends, and even met a few people who are also HG+ among her classmates, even if she is pretty singular even in this environment. Virtual schools
tend to have a lot of draw for HG+ kids in my state because outside of a single urban area, there is virtually nothing for them in regular schools. So this is about as close to ability grouping as we're likely to get.
On the down side, being 4-7 years younger than one's classmates means that they discuss cultural influences that she simply isn't developmentally ready for. The down side of a virtual school is that it also often functions as an "alternative school" for kids that had
other kinds of problems functioning in regular schools. Struggling students who are doing credit recovery are sometimes in the "basic" version of the same courses my DD is in as an "honors" student-- and may share a lecture section. THAT is not so great, and it sometimes leads my DD to feel a little like an odd duck. But PE is decoupled from the academic environment entirely, so she can hang out with the homeschoolers during the day for that, and get social time with similar-age peers.
I'm not sure it's a perfect solution, and it's one that we've had to make a lot of compromises/sacrifices for, but it works. Sort of, anyway; I mean, it worked for quite a while... which is all I can hope for with DD. You ride the train til the track runs out, right?
