I am only intimately familiar with how it works in Texas but my gemeral understanding is your opinion is misinformed. The standardized testing movement creates a standard for basic competency. Schools are strictly evaluated on the percentage of students who meet this standard. Because schools have limited resources and because there is no incentive for overachievement instruction/ curriculum is puposely designed to get as many children up to that standard NOT to exceed that standard. In short the middle is who is best served by public school.

This is where wealthy public school and SOME private have a leg up everyone else. Schools with huge PTA budgets can hire additional staff and provide additional resources to broaden curriculum and instruction to meet the needs of high achievers. And Private schools (generally unconstrained by testing standards) that are well funded and have strict admissions can produce outstanding results. But I think most private schools are a waste of money and only attract students because of an uninformed community.

Lastly schools receive funding for special needs students from the federal government and are largely exempt from testing thus most public schools do a great job with that side of the bell curve.