A federal lawsuit filed this month accuses Philadelphia’s largest charter-school operator of mishandling an investigation into an alleged sexual assault of a 13-year-old girl by another student.

The lawsuit, filed in federal court by the 13-year-old’s grandmother, accuses Mastery Charter Schools of failing to employ “even the most basic of safeguards” to protect students. It refers to previous instances of sexual assault and harassment at Mastery’s Pastorius-Richardson Elementary before the May 2016 incident that spurred the suit, as well as a federal civil rights investigation.

Mastery leaders said that one of the prior incidents mentioned in the suit did not happen, and that the others occurred before Mastery took over Pastorius-Richardson.

According to the lawsuit, first reported this week by WHYY:

The 13-year-old student was sexually assaulted in the Pastorius-Richardson auditorium by a 15-year-old student, who video recorded the girl without her knowing, and the recording then circulated among students and staff.

School leaders “callously dismissed” the incident as consensual, though the girl was only 13 and said she had told the boy “no.” Jason Pearlman, a lawyer for the grandmother, said the school suspended the girl and the boy after the incident.

Now in 10th grade at Martin Luther King Jr. High School, the girl has suffered post-traumatic stress and major depression, and has required emergency treatment, according to the lawsuit, which seeks unspecified damages.

“The manner in which Mastery handles sexual assault and harassment at its schools is outrageous and fails to provide even the most basic of protections for young girls," Pearlman said. "The students, parents, and the community deserve and should demand much better of Mastery.”

In 2017, a dean at one of Mastery’s schools was charged with statutory rape involving a 14-year-old girl.

Mastery CEO Scott Gordon said Tuesday, “I completely disagree” that Mastery — which operates 24 charter schools in Pennsylvania and New Jersey with 14,000 students — has a systemic problem with sexual assault protections.

“Mastery does a wide variety of training with our staff regarding sexual discrimination, youth development, race and identity,” Gordon said.

Pearlman said that at the time of the May 2016 incident, Mastery was "wholly deficient” in its Title IX procedures. The federal law prohibits discrimination on the basis of sex in education programs that receive federal funding.

A federal Office for Civil Rights investigation found that both the sexual encounter between the students and the nonconsensual video recording “should have triggered an investigation under Title IX,” including determining whether the video created a sexually hostile environment for the girl. Neither the principal nor the assistant principal followed such procedures, according to the office.

Mastery said it did not invoke Title IX because it had determined the encounter was consensual, based on the video and information obtained from the students and family members. It noted the Civil Rights Office’s finding that “the school conducted a prompt and equitable investigation of the incident."

The office reached an agreement with Mastery in December 2017 to bring the charter operator into compliance with Title IX. Gordon said Tuesday that Mastery had fully implemented the agreement.