Objective: To examine the developmental trajectories of school physical activity, depressive symptoms, peer relationships, and aggression among junior high school studnets and to explore their interrelationships, so as to provide evidence for promoting mental health and social adaptation among junior high school studnets. Methods: A longitudinal study was conducted among 333 first year junior high school students from two middle schools in Shanxi Province, China. Participants were recruited using convenience sampling in October 2022 (T1) and were followed up in January (T2) and June 2023 (T3). School physical activity was assessed using a modified version of the International Physical Activity Questionnaire (IPAQ). Depression was measured using the Center for Epidemiologic Studies Depression (CES-D) Scale, while peer relationships and aggression were assessed using the peer nomination method. Pearson correlation analysis and latent growth curve modeling (LGCM) were performed to examine the developmental trajectories and their associations. Results: School physical activity,depressive symptoms, peer relationships, and aggression were all positively correlated across the three measurement occasions (T1, T2, and T3) ( r =0.39- 0.83 , all P <0.01). LGCM indicated that school physical activity increased over time (intercept=469.79, slope=0.64). Depressive symptoms (intercept= 1.36 , slope=0.57), peer relationships (intercept=-0.08, slope=0.63), and aggression (intercept=0.65, slope=0.15) also showed increasing developmental trajectories. Multivariate LGCM further demonstrated that both the initial level and growth rate of school physical activity significantly predicted lower initial levels of depressive symptomsand a slower increase in depressive symptoms ( β =-0.17, -0.11), while positively predicting higher initial levels and faster growth of peer relationships ( β = 0.17, 0.10) (all P <0.05). However, school physical activity did not significantly predict the developmental trajectory of aggression ( P >0.05). Conclusion School physical activity is associated with reduced depressive symptoms and improved peer relationships among early junior high school studnets, whereas its predictive effect on changes in aggression is not significant.