Namenic,
Most people will agree that earlier intervention is better - (like cakes’ example from France). Have you considered that additional resources for bottom set/table may be more efficient than spreading these over the whole class? Does the targeted intervention in France counts as a ‘set’?
What French policy seem to be is to enable some children to mature, to develop the necessary neurocognitive skills in Early years, in a formal compulsory preschool curriculum before they enter into the the proper Primary school. Their data show that this reduced the number of children who get behind vs peers in class at the age of 6 and 7, which was the biggest problem, but also reduced the number of children who can't keep up throughout the school career. The intervention as it were is to retain selected children in early years to help them to develop to the level at which they would cope better in school. It is a structured early years curriculum, not baby sitting.
The disadvantage for summer borns throughout the school career is well documented. But there is a broader scope for differences in maturity and neurocognitive 'readiness' - children on autism spectrum and other neurodevelopmental disorders for example.
It is a matter for neurocognitive scientist, but it is well known that huge development takes place in the brain between 4 and 7. Language , executive functioning for example. Certain mix of cognitive skills is necessary to cope well, to keep up in mixed ability education. The French seem to keep some children in EY before they develop this mix.
French 'normal' year of starting primary school is 6. But their policy seem to be to enable some children to start first year of primary school at 4, some at5, at 6, at and even at 7 (1.2%). As a result all the year groups are mixed age, but might actually have a more narrow spread of 'ability'. The class constituted like this is a set of sort, and it copes better with the pace of the curriculum throughout. Maybe a small fraction of the most able can also skip a year forward, I am not sure, there is a handful of 15 years olds starting university, though I am not suggesting to do this.
The result of this policy seem to be rapid, drastic reduction in the number of children who can't keep up with the mixed ability class.
The other result is they were able to increase the pass rate at 15 to 90%, and at 18 to 80% of population. It resulted in better outcomes for more children. They still have a smaller group of 7% of children who fall behind at some stage of school and may even leave school at 18 without any qualifications (Clavinova's 100k), but this is a smaller minority (10%-12.5% of population) then before.