The victim, identified as former Kelvin student Brett Bourne, became involved in a confrontation during the noon hour with a 16-year-old boy from Kelvin.
When that incident ended and the 16-year-old was going to head back into the school, another 17-year-old boy — also a Kelvin student — attacked Bourne, police said.
Emotions were high and the 17-year-old attacker "made a very extreme decision," police spokesman Const. Jason Michalyshen said.
"[There is] certainly a possibility it was over a girl," he added, saying interviews are still ongoing to determine that.
Violence like that at Winnipeg schools is rare "but it does happen," Michalyshen said.
"Schools are safe places to be. They are. But unfortunately, today, we are relaying some very, very bad news."
No charges have yet been laid against the suspect because police are still working with the Crown prosecutor's office to determine what charge should be laid, Michalyshen said.
"But it will be a homicide-related charge," he said, adding those details will be available by the end of Wednesday or by Thursday morning.
Bourne's death is the city's 12th homicide of the year.
Grief counsellors were sent to Kelvin Tuesday afternoon and were there again on Wednesday to help students cope in the wake of the violence.
"This is a tragic event and this happened at lunch hour, and it's going to affect different people differently," said Winnipeg School Division Chair Mark Wasyliw.
"We recognize that and have counsellors there. It's just a basic support that we need to put in place to make sure that this tragedy isn't compounded by its impact."
Wasyliw said the school's surveillance video has been handed over to police.
Some Kelvin students told CBC News they saw teachers giving CPR to Bourne, while he was lying on the ground.
A memorial with flower bouquets has been set up on the lawn and fence near where Bourne was stabbed just outside the school, in the city's Crescentwood neighbourhood.
Kelvin student Arabella Moore, who placed flowers at the memorial Wednesday, said she knew Bourne.
"He'd always walk us home if it was dark out and like make sure we were safe. When I heard who the victim was it was just ... in shock," she said.
Moore said Bourne went to Kelvin until last year and had been a friend with the suspect at one time.
A trio of girls from St. Mary's Academy, a private girls' school across the street from Kelvin, also stopped to add flowers to the memorial. They didn't know the victim but said they felt compelled as part of the community to bring something.
Wasyliw said there are many measures in place to make schools safe, but "tragically, we're not going to be able to get to every one."
"I mean, all we can do is try to make our schools as safe as possible. We have security cameras, we have student resource officers — there's one stationed at Kelvin — and we have anxiety programming in place and counsellors," he said.
"When we know that there's issues brewing we try and stamp them out before they get to this level."
Kelvin and St. Mary's Academy were immediately put into a hold-and-secure lockdown after the incident, which meant no one could enter or leave the schools.
That was lifted just after 2 p.m., after a suspect was taken into custody.
Statement from Winnipeg School Division
It is with deep sadness that Winnipeg School Division (WSD) informs the community of the loss yesterday of a former Kelvin High School student. Winnipeg Police Services has confirmed Brett Bourne was pronounced deceased Tuesday evening in hospital as the result of injuries suffered during an altercation Tuesday afternoon near the school.
WSD clinical support services immediately attended Kelvin High School on Tuesday and today to provide support for students and staff. Counsellors will continue to provide services at the schools and in the community as needed.
"We have a good school with great students and a terrific staff and it is devastating to our school community to have such a senseless act take place," said principal Jim Brown. "I'm very thankful to Winnipeg Police Services, the Tactical Support Team and Winnipeg Fire and Paramedic Services for their response and support."
Brown said students and staff at the school are shocked and grieving, but coming together to support each other as details emerge from the police investigation.
WSD School Board of Trustees chair Mark Wasyliw is also appreciative of the quick actions of teachers and staff who ensured students were kept safely indoors while police secured the area.
"I want to convey our deepest sorrow and most sincere condolences of the entire Kelvin High community to Brett's parents and family," said Wasyliw.
Wasyliw noted that he had been personally contacted by Minister James Allum (minister of education and advanced learning) who pledged the province's support in this tragedy, as well as expressions of support from Coun. Orlikow (River Heights - Fort Garry ward) and the City of Winnipeg.
Community members in need of further support can contact the service director at WSD clinical support services at 204-786-7841.
Anyone with further information who has not yet spoken to Homicide Investigators is asked to call 204-986-6508 or Crime Stoppers at 204-786-8477.