A former teacher and administrator at the prestigious Potomac School in McLean probably sexually abused more than two dozen students over more than two decades, and school officials made aware of the incidents never alerted authorities, according to an investigation ordered by the school.

The report lays out a much broader pattern of abuse by Christopher Kloman than emerged during the criminal proceedings against him last summer and fall in a Fairfax County court. Kloman, 75, pleaded guilty to inappropriately touching five girls at the school in the 1960s and ’70s and is serving a 43-year sentence.

The report found strong evidence that Kloman molested 26 other former students from the mid-’60s to the early ’80s. Investigators determined that school heads, administrators, teachers and board chairs were told directly or indirectly about particular allegations, which ranged from touching to rape. But none of those people had a sense of the scope of the abuse, the report found.

Peter D. Greenspun, an attorney for Kloman, declined to comment on the allegations.

Potomac School officials also declined an interview request, but the head of the school, John Kowalik, who started in August, wrote in a letter to The Post that he was disturbed by what the law firm of Debevoise & Plimpton had found after interviewing 240 members of the school’s community.

Christopher Kloman (Courtesy Fairfax County police)

“I am both grieved and outraged by what they experienced,” Kowalik wrote of the victims. “It is extremely troubling to learn that [s]chool leaders were aware of even one allegation of abuse and did not move forcefully to protect our students.”

The investigation also turned up “credible allegations” that three other former employees engaged in misconduct that included inappropriate sexual behavior with students but were never reported to the school.

In another instance, investigators determined that the school should have notified police after an employee was fired for sexual misconduct with a student.

Kowalik and the board chair, H. Lawrence Culp Jr., wrote in a separate letter to the Potomac community that the school would turn over information about all four cases to authorities.

After Kloman’s sentencing in October, seven victims held a news conference, calling on Potomac to provide an accounting of the number of victims and what school officials knew about the abuse.

Kloman was employed by Potomac from 1965 until 1994, serving as a math, history and geography teacher and head of Potomac’s intermediate school. The report found most of the abuse occurred earlier in Kloman’s tenure.

Investigators found the abuse was reported to two heads of the school in the 1970s and that two or three subsequent heads had concerns or heard rumors that Kloman may have behaved inappropriately with students in the past. Kloman was directed to attend counseling after both direct reports.

“With the benefit of hindsight, this step was grossly inadequate,” Kowalik and Culp wrote in the letter.

In the ’90s, the report found, the head of school learned there was an earlier allegation of misconduct by Kloman and that incident was likely an “unstated factor” in the decision to terminate him, along with his inadequate performance.

There was no evidence school officials notified potential future employers of Kloman’s misconduct, and Kloman went on to work at other schools. A board member also wrote a letter of recommendation on behalf of Kloman, although the board member told investigators he was unaware Kloman’s misconduct was sexual in nature.

The decades-old allegations of abuse came to light after one of the victims saw Kloman in the hallway of her son’s school roughly 40 years after she was abused. She reported Kloman in 2011, touching off the police investigation.

Gloria Allred, who is representing a number of the victims, declined to comment.

Kowalik and Culp said the school was taking steps to avoid similar incidents.

The letter said staff had been trained to spot signs of sexual abuse and background checks were or would be performed on all employees, tutors and parent volunteers.

In addition, the school would formalize policies on employees terminated for sexual abuse or misconduct, highlighting it would not provide letters of recommendation or references for these former employees.