#1 was in private schools with class sizes of about 6 per grade (often fewer) for a while. I expect it is always highly-dependent on the staff and administration. In one of the schools, the administration did not place any obstacles, but also was not equipped to mandate that teachers individualize, so our experience varied by the teacher. The other school, the administration was willing (even enthusiastic), and implemented a grade skip on their own (would have been nice to know about it beforehand, but we certainly had no objection), but individualized beyond that mainly on the teachers' initiative. The teachers, however, were delighted to make adjustments, and even made placement changes mid-year, and proposed SSAs.
I think it's really critical to make somewhat lengthy site visits to the prospective school, as the kind of adaptability and pedagogical openness that you will need is difficult to assess based on a website, or their documents.
