OTTAWA -- The Supreme Court of Canada will not hear an appeal from a young offender who was sentenced as an adult in a mass shooting at a northern Saskatchewan school.

Randan Dakota Fontaine was two weeks shy of turning 18 when he first killed two of his cousins at a home in the remote Dene community of La Loche in 2016.

He then went to the school and opened fire, killing a teacher and a teacher's aide and wounding seven other staff and students.

He pleaded guilty to first-degree murder, second-degree murder and attempted murder and was sentenced as an adult to life in prison with no chance of parole for 10 years.

Saskatchewan's Court of Appeal rejected Fontaine's bid last year to be sentenced as a youth.

The province's courts had opted to extend the young offender publication ban on Fontaine's identity while he appealed his adult sentence.

The decision by the Supreme Court on Thursday means Fontaine has no avenues left for appeal.

His lawyer had argued that the sentencing judge did not fully consider Fontaine's low IQ and that Fontaine suffers from fetal alcohol spectrum disorder.

The Crown has said the shooting displayed sophisticated planning.

Court heard Fontaine was a quiet teen who had friends, but didn't like school work and was attempting Grade 10 for a third time.

He was being raised by his aunt, his adoptive mother, and had no complaints about his upbringing and denied being bullied at school.

Court heard Fontaine researched guns and did an online search the night before the shooting that asked, “What does it feel like to kill someone?”

He had also been playing video games that night with Dayne Fontaine, 17, and Drayden Fontaine, 13 - two cousins whom he called his brothers. The Canadian Press had not reported that relationship previously because of the young offender publication ban.

The cousins lived across the street with the shooter's grandparents and he visited frequently, sometimes between five to 10 times daily.

All three boys went to school the next day, Jan. 22, 2016.

At school that morning, the shooter looked up websites on his cellphone about the Columbine High School shooting in the United States.

After going home for lunch, he went to his grandparents' house. He got a .22-calibre rifle from Dayne's room and called Dayne down into the basement.

That's when Randan Fontaine opened fire. Dayne ran upstairs. Fontaine kept firing, hitting Dayne in the back of the head, and Dayne fell to the floor.

Court heard Dayne tried to cover his head and told his killer he didn't want to die. He was shot again - a total of 11 times.

The younger brother, Drayden, who was outside looking for a ride back to the school, soon after followed Fontaine back into the house and was shot in the head.

Fontaine next returned to school, armed with a shotgun and ammunition in his pocket.

He fired at students in the foyer of the main entrance. Then he walked into the office and shot 36-year-old teacher Adam Wood. Wood died in hospital.

As staff and students ran screaming and hid in classrooms and under desks, Fontaine shot and killed Marie Janvier, a 21-year-old teacher's aide, who was born and raised in the community.

Soon after, RCMP found Fontaine hiding in a school washroom.

He came out and told officers, “I'm the shooter.”