There's plenty of data showing that time spent in school does not strongly correlate with learning, and that kids that start school much later than their peers very quickly catch up to those peers [1,2]. School is mainly subsidized daycare, but it's daycare that's important especially for poor kids.
[1] https://astralcodexten.substack.com/p/kids-can-recover-from-...
[2] https://astralcodexten.substack.com/p/highlights-from-the-co...