ultramarina, when it comes to K writing, I've seen a wide range from posh private schools to Title I schools to affluent public schools since one of local malls used to do an exhibit of Mother's Day letters by K students from all over the city. Your son's writing would be considered advanced for K in any settings except at GT schools where it might not stand out one way or another.

Overall, if you think your DS would be a good fit at the magnet school without skip, then the wise strategy seems to be advocating for subject acceleration for the remainder of this school year as well as next year.