The group has repeatedly called for an independent investigation of sexual abuse that came to light in 2012. Since The New York Times first reported on the school’s legacy of sexual abuse, Mr. Brooks says 63 victims and 22 faculty perpetrators have been identified.

A letter from Mr. Friedman announcing his decision to students and others did not explicitly mention the abuse.

Asked for comment, Horace Mann’s Headmaster Thomas Kelly replied with that letter.

In an interview with the school’s student newspaper, Mr. Friedman said, “Trying to get a resolution to the best of our ability and to be as fair as possible to the number of people that we had to deal with was very hard. We did it methodically, slowly, but it took an enormous amount of time.”

Mr. Friedman, a class of 1972 graduate who chaired the board for the past nine years, added that he “didn’t really sign up for the challenges” that sprang from the revelations of abuse.

Outrage continues

The school’s response to the scandal outraged many of its alums. Though the school issued a public apology in 2013 — nearly a year after the Times article was published — many called the message “disingenuous,” as the school declined, and has since continued to decline, to investigate the abuse.

“There’s been very little compassion on the part of Steve Friedman and the apparent actions of the board,” Mr. Brooks said.

In October 2013, a letter circulated among victims of Horace Mann sexual abuse called for Mr. Friedman to either resign or create a more transparent process in handling the abuse revelations. But Horace Mann’s Alumni Council quickly voted against removing any of its leadership.

Following Mr. Friedman’s retirement announcement, Mr. Brooks expressed some hope that the board’s new leadership could fix Mr. Friedman’s perceived mistakes in dealing with the allegations. Mr. Friedman will be succeeded by the board’s current vice-chair, Michael Colacino, a 1975 graduate.