Toronto police say they're investigating an alleged sexual assault involving students at a Catholic boys' private school, where, according to school officials, some pupils have already been expelled.

St. Michael's College School said Wednesday it "promptly conducted an internal investigation, including meeting individually with the students involved and their parents.

"As a result, swift and decisive disciplinary action has taken place, including expulsions," the school said in a statement.

"The school has zero tolerance for such behaviour."

Local media say students have reported seeing a graphic video of the assault, which was shared on social media and later removed.

Investigators said Wednesday they believe a video of the alleged assault is circulating, and warned it qualifies as child pornography.

"The video must be deleted immediately and cannot be shared with anyone," police spokesperson Meaghan Gray said in a statement. Police say the investigation is with officers from 13 Division and the Child and Youth Advocacy Centre (CYAC).

The school is known for its athletic program; alumni include hockey greats Frank Mahovlich, Dave Keon and Tim Horton.

'Very serious incidents'

Principal Greg Reeves, in a separate email to parents, referred to "two very serious incidents" that occurred on campus and were in violation of the student code of conduct.

Reeves said the administration was informed of them on Monday, and addressed the student body in a formal assembly Wednesday. The school teaches Grades 7 through 12.

"Our concern is first and foremost with the safety and well-being of our students and we are shocked and heartbroken that such incidents have taken place at our school," Reeves wrote to parents.

A follow-up email, from the director of student affairs, said additional support is available for students who need it.

Given police involvement and the privacy of those involved, the school said it will not be making any further comments.

With files from The Canadian Press