. . . Last year, the College Board, the nonprofit corporation that controls all the high-school Advanced Placement courses and exams, published new guidelines for the AP U.S. history test. They read like a left-wing dream. Obsession with identity, gender, class, crimes against the American Indian and the sins of capitalism suffused the proposed guidelines for teachers of AP American history. As of a few weeks ago, that tilt in the guidelines has vanished. The College Board’s rewritten 2015 teaching guidelines are almost a model of political fair-mindedness. This isn’t just an about-face. It is an important political event. The...