Mexican President Enrique Peña Nieto, center, at the announcement of an education reform plan.President Enrique Peña Nieto on Monday announced a broad reform aimed at improving Mexico’s sclerotic educational system, a plan that also seems aimed at weakening the enormously powerful teachers union. Mexico has some of the worst public schools in the hemisphere, consistently placing low on international surveys of education quality. Many blame the union and its autocratic boss, Elba Esther Gordillo, who have perpetuated a system that allows teachers to buy or inherit their jobs, regardless of skill or qualifications. The initiative announced Monday by Peña Nieto...