When I moved to Durham, North Carolina, in the mid-1980s, the county had two separate school systems. At its center, like a bulls-eye, was the city system, which was overwhelmingly African American and had the state’s highest dropout rate and some of its lowest test scores. Its undersized tax base, including a hollowed-out downtown, made it hard to raise enough revenue to close the gap.

Encircling it was the county system, which was whiter and more suburban, and it included Research Triangle Park…