This study examined the development of school-related stress across two student cohorts in Sweden: one educated under a less performance-oriented system, and the other under a more performance-oriented system characterised by earlier grading and increased assessments. Drawing on longitudinal survey data and register-linked sociodemographic variables, growth mixture modelling identified distinct trajectories of stress development, from school year 6 to school year 12. The findings reveal both shared and cohort-specific stress patterns, with stress levels converging by the end of upper secondary school despite divergent early trajectories. Gender, socioeconomic background, and migration background predicted trajectory memberships, though the strength of these associations varied between cohorts. Girls, as well as students from lower socioeconomic backgrounds regardless of gender, in the later cohort were more likely to belong to high-stress trajectories, suggesting that reforms between the two cohorts may have exacerbated disparities on school-related stress.