BACKGROUND: Social background and birth characteristics are generally found to be independently associated with school achievements but the underlying mechanisms are not fully understood. This study aimed to explore how parental education and shorter gestational age jointly influence school performance in a cohort of Swedish children. METHODS: 10,835 children born between 1973 and 1981 were studied, the third generation of the register-based Uppsala Multigenerational Birth Cohort. Ordinal logistic regression models were fitted to estimate OR of achieving middle and high grades in Swedish language at age 16 years, relative to low grade, by parental education and own gestational age, adjusting for potential confounders. RESULTS: In children from families with lower parental education, the adjusted OR of receiving a higher grade was 0.54 (95% CI 0.41 to 0.71) for preterm (<37 completed weeks) compared with full-term births. This estimate did not change when adjusted for several potential confounders (0.59; CI 0.44 to 0.79). When different cut-points were selected to define preterm birth, the estimated OR for those with low parental education decreased linearly from 0.83 (CI 0.72 to 0.96) using less than 39 weeks as the cut-point, to 0.52 (CI 0.30 to 0.90) using less than 35 weeks. There was no evidence of significant effects of shorter gestational age for children with parents from other educational groups. CONCLUSIONS: The disadvantage of shorter gestational age on the chance of achieving higher grades in Swedish language was confined to children from families in which none of the parents had higher education. This suggests that the detrimental influence of shorter gestational age on school performance in language may be avoidable.