I think you’re overestimating how much actual education takes place each day. Most kids can catch up fine on double the workload after some extended break even without in class lectures. Just abstractly the extreme example is someone skipping a full grade, but consider the huge middle ground between that and needing to be in class essentially every single day.

That said a significant fraction of kids really do need all the help they can get, but catering to them means leaving a lot of slack in the schedule.