Background: The main objective of this study was to investigate the interaction effect of school socioeconomic composition (SEC) and adolescent socioeconomic status (SES) in the prevalence of overweight and obesity among a representative sample of French adolescents of the third most populous département of France. Methods: 1038 adolescents agreed to participate (response rate: 91.4%). They self-reported anthropomorphic variables, SES, school lunch and physical activity. The body mass index was divided into six categories according to the Center for Disease Control. Multivariable binary logistic regressions analysis without and with interaction term were performed on overweight or obesity. Models fit was compared using the Aikaike Information Criterion. Oddsratios (OR) and their 95% accelerated-bootstrap confidence interval (95%BCa CI) were computed to estimate overweight or obesity risk. Results: 8.9% of the adolescents were overweight. 3.4% were obese. No school-SEC effect was observed among low-SES adolescents. Medium-SES adolescents were at greater risk in low-SEC (OR = 10.75, 95%BCa CI = 2.67-64.57) and medium-SEC (OR = 5.08, 95%BCa CI = 1.55-24.84) compared with high-SEC schools. High-SES adolescents in low-SEC schools were at greater risk compared with those in medium-SEC (OR = 5.94, 95%BCa CI = 1.94-17.29) and high-SEC schools (OR = 4.99, 95%BCa CI = 1.71-13.14). A social gradient was observed in medium-SEC (OR low/high = 2.79, 95%BCa CI = 1.22-7.41) and high-SEC (OR low/medium = 6.86, 95%BCa CI = 1.06-5.22*10 6) schools. Conclusions: Physical activity and lunch at and outside school help to understand these differences. Implications for obesity prevention initiatives are discussed.