Mr.Greene is a senior fellow at the Manhattan Institute’s Education Research Office. Buried in a new report by the U.S. Department of Education is a comparison claiming to show that charter schools — independently run public schools free from many restrictions — have lower average test scores than regular public schools. A front page New York Times story, put together with the help of America’s second-largest teachers union, recently trumpeted this previously obscure statistic. But the original report buried the finding for a very good reason. Such a broad comparison between charter schools and regular public schools is sheer nonsense....