The biggest problem with Benedikt’s argument is the fallacy of composition–of mistaking the part for the whole. We are willing to stipulate that improvements to the public schools are a common good–that all else being equal, better public schools would make everyone better off (although the benefit would be far from equally distributed).
But a common good is not the common good. Benedikt concedes that parents who follow her advice would be consigning their children to “mediocre educations.” If that simply meant that the school experience would be less pleasant or personally fulfilling, then it would be accurate to characterize this as a personal sacrifice that would not diminish the common good. But if that were the case, it wouldn’t enhance the common good either–only the private good of public-school students whose educations would be marginally more pleasant and fulfilling.
The assumption behind treating education as a public good is that in general, educating children makes them more successful adults, and successful people are more valuable to society than unsuccessful ones. If that is true, then consigning your child to a mediocre education is harmful to the common good, because it reduces his likelihood of success–which can mean everything from becoming a gainfully employed taxpayer to discovering a cure for cancer.