The huge elephant in the room is the our education system isn't built or designed to be in the best interest of children. School does a few things very well - it frees parents from needing to be available for their children during working hours, it conditions students to accept obedience-based hierarchies, and it makes it much easier for bureaucracies to interact with the public as a whole. None of these are central needs of children.
If holding your children back for one year is good, how about twelve?
If holding your children back for one year is good, how about twelve?