Background To investigate whether school performance is a risk factor for suicide death later in life and, if so, to what extent this is explained by intergenerational effects of parental education.

Methods This population-based cohort study comprises national birth cohorts between 1972 and 1981 in Sweden. We followed 898 342 students, graduating between 1988 and 1997 from the 9 years of compulsory school, equivalent to junior high school, until 31 December 2006, generating 11 148 758 person-years and 1490 suicides. Final school grades, in six categories, and risk of suicide were analysed with Poisson regression.

Results The incidence rate ratio (RR) for suicide death for students with the lowest grades was 4.57 (95% CI 2.82 to 7.40) for men and 2.67 (1.42 to 5.01) for women compared to those with highest grades after adjustment for a number of sociodemographic and parental morbidity variables, such as year of graduation, parental education, lone parenthood, household receiving social welfare or disability pension, place of schooling, adoption, maternal age and parent's mental illness. Students with grades in the middle categories had RRs in between. These relationships were not modified by parental education.

Conclusions The strong association between low school grades and suicide in youth and young adulthood emphasises the importance of both primary and secondary prevention in schools.