My DS7 skipped 1st grade and then moved mid-year 2nd to an accelerated school for HG kids, and the resulting 2 year acceleration is working well right now. If we had stayed in the local school, we could have found a way to make it work, but because DS's handwriting was pretty bad (school's reason), most of his math acceleration had to be done in pullout format (rather than joining the higher class). The writing is still tough for DS now in 3rd, having missed a year of writing in 1st, but he's doing pretty well.
We are lucky to have an option of a public HG school relatively near by, so that DS is on average only a year younger than the kids in his class. I think if we had stayed in our local school, DS would need another skip pretty soon. The trouble is usually pacing in elementary school. Even if your kiddo is getting more appropriate subject matter, the pace of the class (unless a GT cluster) will be slower. That said, it's still better than material that your kiddo already knows, and since your school sounds very flexible (most important thing), then they will hopefully also be willing to differentiate.
Speaking of differentiation, a really good thing to look for with your flexible school is placement with teachers who are good at differentiation, and who want your kid in their classes.
As for the social bit, it probably depends on the personality of your kid. But remember there is a huge range of acceptable behavior/maturity levels in each grade. As long as your kid falls into that range (even if on the low end), then I wouldn't see a problem in that alone.
