Sex education is a sensitive subject. But during nine years at the prestigious Dalton School on the Upper East Side of Manhattan, seven of which she spent as the director of health and wellness, Justine Ang Fonte seemed to be handling it with success. She developed curriculums for students from kindergarten through 12th grade; hired three other health educators; and organized documentary viewings, discussions and workshops for parents. She also was a regular speaker at educational forums, like the National Association of Independent Schools’s People of Color Conference, and offered workshops and presentations at other New York City schools. One...