It seems to me that they could just give the kids some assessments, see how they do, and move ahead.
You are advocating tracking (perhaps more fairly described as ability or readiness grouping) starting in kindergarten. In our school system, like many others, tracking only starts in 7th grade in math and 9th grade (high school) in all academic subjects. When I was in school there was ability grouping in math and reading in elementary school, so ability grouping starts later than it used to.
I support your proposal, but elementary school teachers and administrators want to keep age-mates in the same classes studying the same things, with the pixie dust of "differentiated instruction" solving the problem posed by heterogenous grouping.
