I am grateful that my children's school has consistently used tracking starting in 1st grade even though the district GT curriculum does not start until 3rd grade. There is some shuffling between 1st and 3rd grade but the majority of kids have been together in the same class since 1st grade. It is simple to implement and beneficial to the kids in the top 15% or so.

I think that intra-class grouping is much trickier to implement and I have only seen it done well back when I was in elementary school and tracking didn't start until 7th grade.