The magazine, in a story posted online Wednesday, said the school did not inform police or parents of the sexual abuse by at least three teachers, all of whom are now dead, from the 1970s through 1994.
The article, written by 1982 Horace Mann alumnus Amos Kamil, said two of the teachers had been ousted quietly and moved on to other schools.
The magazine said a former trustee at the school told Kamil: "No one will talk to you. They are all lawyering up."
Thomas Kelly, the school's current headmaster, sent parents a letter Wednesday telling them the story was being published.
"These allegations are highly disturbing and absolutely abhorrent," the letter stated. "We can assure you that none of the individuals mentioned in the article is currently employed by the school nor have they been for a number of years."
One former student said Mark Wright, who had been an art teacher and football coach at the school, had asked him to sit for a portrait.
"I remember exactly what he said: that he needed to see the connection between my legs," the former student said. "The next thing I knew, he had my penis in his hand. I was so scared. He was a pretty intimidating guy. He began performing fellatio and masturbating."
One alumnus started a Web site last year about former music teacher Johannes Somary, who he said abused him in 1973. Two decades later, in 1993, the family of then-student Ben Balter said Somary had assaulted him on a trip to Europe.
Balter committed suicide in 2009. His mother said a lawyer affiliated with the school had told her unless she had evidence on tape, she could do nothing.
"It was Ben's word against Somary's," she said she was told.
Stanley Kops, a former history teacher who also coached the swim team, resigned in 1983 following a student's allegation the teacher had touched him on a camping trip. Kops committed suicide the next year.