Doesn't that depend on what school is for? If the importance of school is overwhelmingly the subject matter or the grades, then I'd agree that high-performing children need less help. To the extent that children need something else from school, high-performing children might need more help than average-performing children. Maybe the average-performing children are more dependent on effective study techniques, while the high-performing children can perform at the expected level with little study. If the subject matter turns out later to be useless, at least the average-performing children have picked up the study techniques they need to learn something useful, but the formerly high-performing children might then face a bigger challenge. Even if school subject matter is useful, that doesn't help those who already know it; they aren't gaining knowledge or productive habits.

Sure, in the ideal world school would be a productive experience for everyone.

In the current US system, it's hard to justify focusing additional resources on students that are already performing well.