A teen accused of fatally stabbing 17-year-old Brett Bourne at Kelvin High School in 2015 started his second degree murder trial today. 

In their opening address to the jury the prosecution alleged the accused — who was a youth when Bourne died, and cannot be named — slashed himself with a utility knife after the killing in an effort to claim the victim had been armed with a weapon.

The now 19-year-old accused has pleaded not guilty to second-degree murder in the June 2015 killing.

The stabbing was sparked by a dispute between the victim and another teen involved with the victim's ex-girlfriend, Crown attorney Krista Berkis told jurors Monday.

Berkis said Bourne was cycling past the school during the lunch hour when he saw his ex-girlfriend's new boyfriend on school property and tried to goad him into a fight.

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As other students gathered, the accused retrieved a knife from a friend's car before Bourne chased the new boyfriend into the school, Berkis alleged.

Police at Kelvin

Police block Kingsway in front of Kelvin High School. (John Bronevitch/CBC)

"At some point, while inside the school, the accused stabbed Bourne one time in the side of the chest," Berkis said.

Bourne stumbled outside where he collapsed on the sidewalk. 

Police arrived at the school to find two people performing CPR on Bourne.

"His eyes were open , they were rolled back," Const. Karen Hood testified. "There was no breathing and he was unresponsive. I called for an ambulance on a rush."

Hood said she saw no weapon at the scene.

Flowers laid at Kelvin High School

A woman who asked not to be identified puts down flowers on the lawn of Kelvin High School. Her daughter witnessed the stabbing. (Trevor Brine/CBC)

Bourne was rushed to Health Sciences Centre were he was pronounced dead an hour later.

Berkis went on to claim that following the stabbing, the accused was driven to a friend's home where he cut his left forearm with a utility knife and cleaned the knife he used to stab Bourne. The accused was then driven home where he changed out of his blood-spattered clothes.

The accused then walked back to school with his mother "and showed an officer at the scene his self-inflicted cut, but he told the officer the deceased had done that to him," Berkis said.

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The trial is set for four weeks and is expected to hear testimony from several student witnesses, including one who captured part of the violent exchange on video.

Berkis said the incident was traumatic for some of the students who witnessed the stabbing, and cautioned the jury to "remember that many of them were teenagers at the time of the incident and also that it happened on their school grounds."

"These students are now adults, but keep in mind that they are still very young adults and that is something to take into consideration when listening to their evidence."