That effect, known as "suicide contagion," can last two years or longer, researchers reported Tuesday in the Canadian Medical Association Journal.
The findings are based on 1998-2007 data from a biennial Statistics Canada survey of more than 22,000 children aged 12 to 17 from across the country.
Researchers found that the suicide of a schoolmate magnifies the risk of suicidality for adolescents, even if they didn't personally know the young person who died.
"One of the questions they were asked was: 'Have you seriously considered attempting suicide in the last year?'" said senior author Ian Colman, Canada Research Chair in Mental Health Epidemiology at the University of Ottawa.
"And that's a pretty serious indication of suicidal thinking. These aren't casual thoughts."
The contagion effect was most pronounced among the youngest survey respondents. The study found 12- to 13-years-olds were five times more likely to have thoughts of ending their lives if a schoolmate died by suicide compared to those who weren't exposed to suicide (15 per cent versus three per cent).
Among those in this age group, 7.5 per cent attempted to take their own lives after a schoolmate's suicide compared with 1.7 per cent who had not had a peer die that way.
Teens 14 to 15 were almost three times more likely to have suicidal thoughts, while 16- to 17-year-olds were twice as likely.
Researchers don't know why those 12 to 13 are so vulnerable, but Colman suspects the baseline proportion of youngsters that age contemplating suicide is low.
"Many of these children, they've never thought about suicide — or never seriously.
"And then suddenly they're confronted with a death by suicide in their school and it's the first time that they've had to think about suicide and the consequences," he said. "And for the first time in their life, they might actually think about suicide as a solution to their problems.
"And that's exactly what we're trying to avoid."
The researchers also were stunned to learn that by the age of 16 to 17, one in four teens surveyed had a schoolmate who had died by suicide.
"We were absolutely shocked that the proportion of children that would report that someone in their school had died of suicide would be that high," said Colman.
As well, one in five said they personally knew the student who had died, although the researchers could not determine how close the relationships were.
It's also not known why the contagion effect persists so long, he said. "All we're able to say with any conviction is there is clearly an association.
"There is no doubt that students who report that someone in their school had died of suicide are much, much more likely to be thinking about suicide themselves and are much more likely to be thinking of suicide as long as two years later."
Child and adolescent suicides are still relatively rare: 2009 Statistics Canada figures show 227 Canadians aged 10 to 19 died by their own hands, with 202 of those cases were among 15- to-19-years-olds. But as the second leading cause of death after accidental injury among Canadian youth, it's an issue that needs to be taken seriously, he said.
The study findings suggest that school-based programs to prevent subsequent suicides, often called "postventions," should focus on all students and not just target those who were closest to the young person who ended his or her own life.
"And schools might want to consider longer-term interventions or revisiting their programs or their interventions over time to make sure children are still coping well," said Colman, stressing that short-term assistance plans may not meet children's needs.
Schoolmates of Rehtaeh Parsons would be among those who need to be closely monitored, he agreed. The 17-year-old Halifax girl took her own life last month following an alleged rape and online harassment that included posting photos of the assault.
Her family has said they couldn't get Parsons the help she needed to deal with months of relentless cyber-bullying.
"For school and more broadly if we want to think about suicide prevention," said Colman, "what we need to do is create a culture where students or children feel comfortable seeking help if they're struggling with their mental health.
"That they know they can speak to an adult who's going to help them, that they don't have to be afraid to come forward with these kinds of problems."
In a related CMAJ commentary, Dr. India Bohanna of James Cook University in Australia said the study provides convincing evidence that for young people, exposure to suicide is a risk factor for future suicidal behaviour.
"The idea that suicide is contagious has always been controversial for various reasons; however, this important study should put many, if not all, doubts to rest," Bohanna writes.
"A unified and concerted effort now needs to be directed toward developing evidence-based postvention strategies. We need to know what works in mitigating the risk of contagion and why."