Increasing levels of psychosomatic symptoms, and other mental health problems, among adolescents, and especially among girls, have been reported across various countries. The "educational stressors hypothesis" states that this trend can be explained by an increasing amount of stressors in the school environment. This study tests this hypothesis, using repeated cross-sectional data, between the years 1993-2017, from the Health Behaviours of School-aged Children (HBSC) survey. Regression and decomposition techniques are used to investigate the role of school stress for trends in psychosomatic symptoms, and for gender differences in symptoms. Results show that the effects of school stress on psychosomatic symptoms have become stronger over time, but that they can only account for a small share of the overall increase in symptoms since 1993. However, school stress has increased more among girls than among boys, and it explains about half of the growth of the gender gap in symptoms. Thus, school stress accounts for a substantial portion of the increase in symptoms for girls, but only a minor share of the increase for boys. In sum, we found weak evidence for the educational stressors hypothesis in regard to the overall trend in symptoms, but strong evidence for it in explaining the growing gender gap.