Officials have not given a motivation for the shooting in La Loche, about 600 km (375 miles) north of the city of Saskatoon.
“Obviously this is every parent’s worst nightmare,” said Prime Minister Justin Trudeau, who initially reported five people were killed. He was in Davos, Switzerland, for the annual World Economic Forum.
Mass shootings are relatively rare in Canada, which has stricter gun laws than the United States. In the country’s worst school shooting, 14 college students were killed at Montreal’s Ecole Polytechnique in 1989. A shooting in 1992 at Concordia University in Montreal killed four.
The latest shooting occurred in the high school, called the Dene Building, and another location in Saskatchewan, Trudeau and Canadian police said.
Police took the suspect into custody outside the school and seized a gun.
La Loche acting Mayor Kevin Janvier told the Canadian Press the incident may have started at the suspect’s home.
“I’m not 100 percent sure what’s actually happened but it started at home and ended at the school,” Janvier said.
Among Canada’s provinces, Saskatchewan had the highest rate of police-reported family violence in 2014, double the national rate of 243 incidents per 100,000 people, according to a Statistics Canada report on Thursday.
Extra doctors and nurses were sent to treat patients in Keewatin Yatthe Regional Health Authority’s 16-bed hospital, said spokesman Dale West. He declined to say how many people had been injured.
Teddy Clark, chief of the Clearwater River Dene Nation, said his daughter told him about the shooting, the Canadian Broadcasting Corporation reported.
“We’re just trying to pull together here and make sense of all this,” Clark told CBC television. “It’s not a very pretty scene right now.”
La Loche student Noel Desjarlais told the CBC that he heard multiple shots fired at the school, which has about 900 students.
“I ran outside the school,” Desjarlais said. “There was lots of screaming, there was about six, seven shots before I got outside. I believe there was more shots by the time I did get out.”
A cellphone video taken by one resident and broadcast by the CBC showed students walking away from the school across the snow-covered ground and emergency personnel moving in.
In 2014, a teacher expressed concern about violence at the La Loche school, noting that a student who had tried to stab her was put back in her classroom after serving his sentence, and another attacked her at her home.
“That student got 10 months,” Janice Wilson told the CBC of the student who tried to stab her in class. “And when he was released he was returned to the school and was put in my classroom.”