The classes are never the suitable speed for all the students. When learning on your own, you set your own speed but you can't do that when in school. But note that all subjects are like the subjects you mentioned you took in HS. The mathematics doesn't actually go all that far. Note that all of HS mathematics can be summarized in those HS math review books that are actually not that thick. It's a travesty that fundamentally, you spend maybe like 4 years reading like 6-7 books?
The point is not to be optimal for every student - the resources required for such are not available and MOOCs are not an adequate replacement. The point is to lay a universal foundation in a series of selected subjects while socializing students.
The model of turning a literate child loose in a library and letting them be for six or eight years should not be assumed as universally useful. It's not a good way to produce a shared general foundation for a modern democracy.
Exactly, I can't think of a single subject that actually achieves what it should when taught in the school setting.