We had the same indecisions when sending the older to a gifted magnet school (school only starts from 1st so DD had to attend home school) and it turned out to be a really good decision for us. Agreeing with syobirig regarding sending your kids to the school where needs will be met first and foremost.
In researching schools and advocacy for our HG+ DD, I was under the impression that pariochial schools in general do not accelerate aggressively, have they given you a detailed plan on how they will do this as your 3rd child's needs will be very different from a moderately gifted child.
DD will now be attending the same school as DS, but this may change next school year if it is not a good fit. This school will not grade skip despite 99.9% IQ and overall achievment in WIAT. I am realizing as she progresses through her elementary years, school changes may be in her horizon.
