A probe commissioned by a group of alumni at New York’s elite Horace Mann School has found that more than 64 students were sexually abused by nearly two dozen faculty and staff between the 1960s and the 1990s—far more than an investigation by the district attorney identified. The investigation, led by former judge and sex-crime prosecutor Leslie Crocker Snyder, said the abusers at the leafy campus in the Bronx included both men and women—and students were sometimes molested by more than one staffer.