Anonymous wrote:if you're worried, jump in during MS years.
Middle school - particularly for girls - is pretty much the worst part of a kid's childhood socially, so I would be very cautious about taking a kid from public and putting them into private for 6-8 unless they're either a) in a position finance/confidence/connections-wise to be one of the 'popular kids' or b) enough of an introvert that people will simply ignore them and they won't mind that.
A high-achieving upper-middle-class public school kid coming in in 6th grade and disrupting everybody's little pecking order might as well be walking around with a giant target on their back.