Intellectual development is not the only kind that happens during childhood, they're also learning valuable life skills, and if you run their lives for them in pursuit of one kind of development you're going to ruin other kinds.

What would you cut from the curriculum, and what would you replace it with?

Public school in the US has English literature, physics, chemistry, biology, algebra, arithmetic, geometry, trigonometry, calculus, second languages, civics, history... That the material exists, whatever you think of it, and kids aren't learning that material, says something about the methodology.

I think sending a kid to what basically amounts to a prison, in an intensely competitive social environment complete with people from all walks of life, including violent people, while simultaneously handicapping anyone's ability to enforce proper conduct, with a strategy geared to basically encourage cheating, is not conducive to learning anything for a long period of time. Then, telling them their time in their life where they get to do real fruitful things like decide their conduct, who they associate with, what interests to pursue, spend time with their parents, let their minds wander, is not theirs and they have to continue the requirements of school, IMO it hampers intellectual development.