You could teach some motivated fraction of children orders of magnitude more academic information but the goal of primary education is equally if not more to socialize them into being what the society demands of its citizens. Seeing foreign elementary schools hammered this home for me, what I’d forgotten or assimilated so much I didn’t even notice it- that teachers are constantly reinforcing their normative values about whats good and bad behavior. Because this varies so much by culture its wild to see for example a Japanese elementary school.