I've been thinking more about this, and another idea popped into my head. - What if we shortened the summer break a little and increased the winter one, and really separated the semesters - so first grade, for instance, would be broken down into 1A and 1B?
Unless a town was so small as to have only one class per grade, they could even start new classes during each semester, so kids could actually begin kindergarten just after they turn five, and we wouldn't have the need for "red-shirting" and the social readiness differences of kids in the same K class being almost a year apart in age.
Mostly, kids would move along with the same teacher for two semesters, finishing on the same schedule they do now - but for kids who are on the borderline of needing to repeat a grade, we would have the option of letting them switch to the other track and only repeat half of the grade to catch up. (The same could go for a kid who might have once been considered for skipping a grade, but for whom a whole year would be too much socially.)
I'm sure it would cause all sorts of issues I haven't thought of yet, so this is purely "discussion fodder" for fun and not an actual goal of mine right now or anything!