ultram, one thing I'd do is to first try to find out what the kindergarten curriculum and classroom experience is really like at this school (if you haven't already). Many of the things you worry about in your OP were never an issue in our kindergarten - reading was broken up into small groups based on ability, math included basic skills that your ds already knows, but it was primarily made up of applied problems so that even though the skills were easy for my kids the actual problems were new.
I have spoken to some other parents in the magnet whose kids have done K and 1. Experiences are mixed--"okay," sometimes "good." What it absolutely isn't going to be is a challenging K classroom like you might find in an UMC area of mostly high-ability kids.
The one question I have is you mentioned the sibs of many of the kids in the gifted magnet school within your school don't go to K-1 grade at the same school - is that because their neighborhood schools are different or is it because their parents are purposely not sending them there?
Both. Very few of the magnet kids are actually zoned for this school. I had to get a special variance to get DS in since it isn't our zone either. Our zoned school is no good at all, so this is a better option.
