HALIFAX -- Student leaders at Saint Mary's University were widely criticized Thursday for performing a frosh-week chant glorifying the sexual assault of underage girls that has been part of orientation activities at the Halifax school for years.

Politicians, school administrators and students said they were shocked by a video of the song that circulated in social media, which spells out the word "young" with a comment after each letter and includes: "Y is for your sister ... U is for underage, N is for no consent... ."

The chant was performed to about 400 students assembled on the football field at the university as part of its orientation week. A video of it was posted on Instagram on Wednesday, causing a deluge of criticism here and abroad.

Jared Perry, president of the Saint Mary's Students' Association, apologized Thursday for performing the chant with 80 male and female orientation leaders.

"We're deeply sorry and we want to turn this around," he told a news conference, as about 20 of the leaders stood silently behind him.

"We've realized we made a huge mistake."

He said they will launch an investigation into the incident and try to determine how the school can prevent sexual assault on campus, while addressing what he referred to as a "culture of sexism" at the university.

Perry added that the same chant has been part of frosh events since at least 2009 when he sang it as his initiation, with the lyrics being passed down on paper to orientation leaders. But he couldn't explain why no one had raised concerns about it previously.

"The fact that this sort of thing was able to happen at this scale and that some (leaders) don't understand the seriousness of it tells me that there is a problem of a culture of sexism that demands attention and real action on this campus," he said.

Several Nova Scotia politicians and advocates for victims of sexual assault voiced their dismay over the chant, which comes in the wake of several recent provincial initiatives to combat non-consensual sex.

Premier Darrell Dexter said he was disturbed by the video, but hoped the school takes a measured approach in its response to the incident.

"You know, kids, they often act without considering, necessarily, the consequences of the things that they're saying," he said. "It was very disturbing."

Irene Smith of the Avalon Sexual Assault Centre said she was shocked to see student leaders condone the assault of young girls only months after the death of Rehtaeh Parsons. The 17-year-old attempted suicide after she was allegedly sexually assaulted by teen boys at a party in 2011.

"The message of that chant reinforces rape culture in our society," she said in a statement.

"The fact that 80 frosh leaders, young men and women, were enthusiastically saying those words on a football field signals that none of them questioned ... the effect and message those words have."

University president Colin Dodds said he was shocked by the video and issued an apology on behalf of the school. He said a senior school administrator and the police met with student leaders prior to the event and raised the issue of sexual assault and sexual consent.

"However, I accept that I and the university administration have a role to oversee and guide student leaders. We failed in that responsibility," he said in a statement.

Dodds said the school has called a special meeting with the executive and board of the student association asking them to explain what happened. The 80 student leaders have been required to take a sensitivity seminar before the end of the month.

Marilyn More, the province's minister for the status of women, said she was dismayed after seeing the video, adding she would have expected people of that age to understand the inappropriate nature of the lyrics.

More said it will take time to change attitudes on sexual behaviour.

"These are certainly young people that we would have thought had given more appropriate thought to the whole issue of sexual violence," she said.

"This has obviously prompted a lot of discussion. I think it's going to lead to even more discussion and action by students around the province. And hopefully these students can turn what was very inappropriate and upsetting into a learning opportunity and move on with their lives."