Over a dozen teenagers are under investigation for a serious of sexual assaults against two Los Angeles high school students, police said Friday.

Investigators believe the assaults took place on an on again, off again basis dating back to the end of 2013 at Venice High School, Los Angeles police Cmdr. Andrew Smith said.

Eight suspects were arrested at the school Friday morning, while a ninth suspect was apprehended off campus. The 10th surrendered Friday afternoon and police are searching for the remaining four.

"This is a painful moment for Venice High School and this district," Los Angeles Unified interim Superintendent Ramon Cortines said, adding that crisis counselors would be stationed at the school to assist students.

Smith said some of the incidents were forced sex acts, while others were consensual, but “the victims were too young to give consent.”

"Incidents like this are a terrible tragedy," Smith said. If there are any additional victims, he said, "We'd ask them to come forward."

The former leader of the school's PTSA said she was unaware of any issues at the school preceding the arrests.

"I've been part of the school for some time, I haven't heard anything whispered even," said Linda Patterson Salib, who was president of Venice High School's PTSA until February. "So this comes as quite a surprise. It's not like, 'Oh we knew there was a problem.'"

The Associated Press contributed to this report