This study examines the relationships between the flow in physical education (P.E.) class, school life stress, and aggression in adolescents. A total of 470 surveys collected from five different high schools in Daejeon, South Korea, were analyzed using structural equation modeling. Study results indicated that the flow in P.E. class had a significant negative direct effect on school life stress, but did not have a significant direct effect on aggression. Furthermore, school life stress had a significant positive direct effect on aggression. Finally, flow in P.E. class had a significant negative indirect effect on aggression mediated by school life stress. The findings of this study are valuable for education experts to establish more effective P.E. curriculums in reducing school life stress and aggression.