In 2012 the Education Department released a national study showing that black students are suspended from school at a higher rate than whites, and the findings fueled a predictable debate over whether school discipline policies are racist. Two years later, the department sent a letter to school districts warning them to do something about the disparity—in effect, to stop suspending so many disruptive black students or risk becoming the subject of a federal civil-rights investigation—and the results have been just as predictable.

The...