Improving adolescents' levels of sport and physical activity (PA) is an official public health issue. French national government plans were launched in 2001, 2006, and 2011 to improve the participation levels of citizens. These plans should be monitored. To date, information on temporal trends in sports has come from the national population. However, no data are available to measure temporal trends in different territories across the country. Our study aimed to measure these trends among a representative sample of adolescent students of the third biggest French region (Bouches-du-Rhône), but also one of the poorest, between 2001 and 2015. Three surveys were conducted in 2001, 2008, and 2015 in high schools (n = 3218). Logistic regressions adjusted for age were used to determine the impact of socioeconomic status (SES) on sports participation and to measure the changes in sport participation rates. Participation declined among all subgroups of adolescents: from 79.0% to 65.8%. The greatest decrease was observed for boys with a high SES, whilst the lowest was for the high-SES girls. We observed that SES inequalities in access to sport increased among the girls, whilst they reduced among the boys. National government plans seem to have had limited success in this territory. Next to national studies, there is a need to develop territory specific studies which could show important disparities across the national territory.