> The problem occurs when you place them all in one school, and force them to learn everything, even things they don't want to learn about

A difficult part is that children aren't really in the position to know what they want to learn most of the time.

Sure, many prefer sports over math but covering a broad spectrum in pre-teen and teenager education is quite important to get them develop these preferences and themselves as a person. They are given more agency/choice (electives etc.) as they grow up.

There are also topics you need to learn that aren't fun/engaging (especially as fun/engaging is quite subjective and depends on the individual). Especially when those topics are prerequisites to other potentially fun topics (you will have to learn the fundamentals before engaging with advanced topics in most subjects)