On to DD6, who is an even murkier case.

DD6 has a birthday just before the cutoff, so was entered at the regular age. Still young for grade and the youngest in her classroom. She is not as "pointy" as DS10, socially more astute and not quite as anxious, but still rather reserved and rigid compared to her age group.
In her case, the school was forced, due to a sudden teacher shortage, to enter a number of first graders into the two second grade classrooms, so she is part of a group of five first graders (three of which, all girls, happen to be redshirted) in a classroom with 24 second graders.
The teacher came up with the grade skip first, and I am again the one who balked. Probably projecting my own traumatizing experience with a grade skip that was, academically, a bill, and socially an unmitigated disaster.

Right now, DD6 is part of this cozy little quartet of first grade girls, and getting more confident in the classroom, and I'd hate to disturb this - but the teacher tells me that with the exception of writing, she could work right along the second graders right now, but is seeing issues with task initiation and completion! Sloppy work and inability to hand I completed work, which I actually think is rather a reason to skip than not, but oh for that crystal ball! The teacher has already started giving her and another girl second grade work, and I have begged her to please do it unobtrusively and to keep sending her to pullout a with the other first graders, at least until the end of the year.

The teacher wants to continue with the split grade classroom next year, so the situation could in theory remain unresolved for a while longer, but does not know whether this will work out, depending on stuff which is out of her hands, like the staff situation next year and what the principal, who is new this year, will decide to do.

If it boils down to a choice between going on into third grade or putting her into a regular second grade classroom come September, we will probably bite the bullet and put her in third.

so, middle school implications? DD6 would be a just turned 9 yo fifth grader. She has expressed a wish to continue at the public college prep school her father teaches at which is right next door to her elementary, and with the acceleration it might work. Alternatively, she follow her brother to the gifted program across towns which would not be as much of a step as it was for him, since we will have moved closer to the school by that time. I do believe it is somewhat easier for a girl than for a boy to rather younger.