The head of St. Paul’s School, the elite Concord, N.H., boarding school that has been rocked by revelations of sexual abuse, announced Thursday that he is stepping down at the end of the 2018-19 school year.

Michael G. Hirschfeld, a faculty member since 1994 and the school’s rector, or head, since 2010, did not mention the scandal in a letter sent Thursday to the St. Paul’s community.

He has been a staunch defender of the school, saying he would always stand up for its faculty, students, and staff even as the New Hampshire attorney general’s office launched a criminal investigation last year to determine whether St. Paul’s administrators engaged in conduct that endangered the welfare of children.

“Leaving the school would be more difficult if I did not know these truths: the school is absolutely on the right trajectory in all things that relate to educating and caring for young people; there are extraordinary people here — talented students from across the globe and faculty and staff who are wholly devoted to advancing our mission; and the time is right for new leadership,’’ Hirschfeld wrote.

In a separate letter, Archibald Cox Jr., the president of the board of trustees, said Hirschfeld’s departure was not unexpected because he had committed to an eight-year term when he was appointed rector seven years ago.

Cox said that, in the weeks ahead, the trustees will “put in place a rigorous process to find the person who will best lead St. Paul’s for the next decade as its fourteenth rector and as we further strengthen the school and its community.’’

St. Paul’s, an Episcopal institution founded in 1856, has a long list of prominent alumni, including former secretary of state John F. Kerry and Senator Sheldon Whitehouse, Democrat of Rhode Island.

But the school has been under a microscope since the 2015 trial of a senior accused of raping Chessy Prout, a 15-year-old classmate, as part of a game of sexual conquest called Senior Salute. The senior, Owen Labrie, was acquitted of rape but found guilty of misdemeanor sexual assault and other crimes.

Then, last May, St. Paul’s released a report that found 13 former faculty and staff members engaged in sexual misconduct with students between 1948 and 1988. The report, by a law firm hired by the school, faulted administrators for ignoring and even concealing the abuse, saying they were often more interested in preserving the school’s reputation than protecting students from assaults and, in some cases, rape by respected faculty members.

Prout said she hoped Hirschfeld’s departure would lead to positive change at the school.

“It’s an opportunity to create a culture where the safety and well-being of students comes first, and to move forward to become a leader among schools in preventing sexual assault and supporting survivors,’’ Prout said.

Alexis H. Johnson, a member of the class of 1976 who was propositioned by two teachers when he was a student, said Thursday the school needs “to adapt to a changed world and, indeed, a world that changed while it was reluctant to change its core values.’’

“St. Paul’s needs to do a whole lot of ego abandonment,’’ said Johnson, a New Mexico lawyer who was appointed to a campus sexual harassment task force in 2001. “You’re no more elite than the weakest among you or the worst habits in your midst. The school ought to address those at it moves forward in the selection of a new rector.’’

Hirschfeld apologized to the school community after the release of the May report, and credited abuse survivors for pushing St. Paul’s to acknowledge the misconduct. He said none of the administrators cited in the report for failing to respond appropriately to allegations of abuse were still working at the school.

In his letter on Thursday, Cox said that he will miss Hirschfeld’s “wise and courageous leadership.’’ Cox said that, as rector, Hirschfeld oversaw the expansion of campus facilities, increased academic professionalism, and enhancements of the school’s culture, “while at the same time having to deal with unprecedented challenges on numerous fronts.’’

Geoff Ward, a senior assistant attorney general in New Hampshire, declined Thursday to comment on Hirschfeld’s decision to step down, but said it would not affect the state’s criminal investigation because it is focused on “the institution and not any one individual.’’

St. Paul’s is one of a number of prep schools that has acknowledged claims of misconduct following a 2016 Boston Globe Spotlight story that reported on allegations of abuse by more than 200 victims at 67 private schools in New England.

Last year, Choate Rosemary Hall in Wallingford, Conn., named 12 former educators who allegedly sexually abused or assaulted students since the 1960s. Phillips Exeter Academy disclosed credible allegations of sexual misconduct by four former employees.

And in 2016, St. George’s School in Middletown, R.I., reported that 51 alumni described being abused by faculty and staff in the 1970s and 1980s. Al Gibbs, an athletic trainer, allegedly abused 31 of the victims.

Michael Levenson can be reached at mlevenson@globe.com. Follow him on Twitter @mlevenson.