A new report commissioned by concerned alumni of Horace Mann School, an elite private school in New York City, tells an extraordinary story of the sexual abuse of scores of students over decades by teachers, coaches, department heads, a headmaster, a school chaplain and a dean of guidance. The findings build on revelations made public in recent years about sex abuse at Horace Mann, which was founded by Columbia University on its campus in 1887 but has long been an independent school in the Bronx, now with 1,782 students from nursery school through 12th grade. It costs $41,900 a year...