A math teacher filmed an 11-year-old boy in a bathroom stall and sexually abused him at a summer program held at the prestigious all-boys Regis High School in the Upper East Side, according to disturbing court filings.

Olaya Lopez, said her son, identified only as “JC” in the lawsuit filed in Bronx Supreme Court Thursday, was molested by teacher Matthew Chicas at the school during the 2023 academic year when the student attended the REACH program, a Jesuit preparatory class for middle school boys.

Chicas, 29, who was a teacher at Xavier High School in Chelsea who participated in the REACH program, was arrested in April and charged with endangering the welfare of a child and attempted unlawful surveillance after the April 15 incident, prosecutors said.

The school never reported the abuse by Chicas and instead tried to suppress it, Lopez also claimed.

However, the boy reported the alleged abuse to another teacher at the school, according to the filing. 

“It’s no secret that there has been a culture of abuse which has been cultivated at this institution,” Howard Shafran, Lopez’s attorney, told The Post. “It goes back many years.”

Shafran pointed to Regis High School’s last headmaster, Rev. Daniel Lahart, who was fired for sexual misconduct in April 2021.

Lahart was ousted after the school’s board of trustees determined he “engaged in inappropriate and unwelcome verbal communications and physical conduct, all of a sexual nature, with adult members of the Regis community, including subordinates.”

Chicas declined to comment on the allegations. Neither Regis High School nor Xavier High School replied to requests for comment.

Regis High School touts notable alumni that include Dr. Anthony Fauci, the former director of the National Institute of Allergy and Infectious Diseases, and Saturday Night Live star Colin Jost.