Rising levels of physical inactivity among children and adolescents, especially among disadvantaged groups, are alarming. In response, schools should implement inclusive, evidence-based, and sustainable interventions that reduce sedentary behavior and promote daily physical activity (PA) both during school hours and in leisure time. The objective of this scoping review is to focus on adolescents with the lowest levels of PA and explore aspects that have been addressed in school-based interventions and components associated with increased PA in sedentary adolescents. Database searches were performed in PubMed, PsycInfo, and Scopus. Interventions had to be school-based, aiming to increase physical activity in participants identified as sedentary adolescents in a typically functioning population. To be included, studies had to provide information about leisure-time PA. Excluded were studies focusing solely on the general population and changes only in psychological or health-related outcomes. Rigorous evaluations of studies fulfilling all eligibility criteria were rendered in a descriptive/narrative analysis of the summarized findings of ten studies. Programs, commonly implemented during physical education (PE) lessons, often relied on PE teachers as key facilitators. Moderate activities such as walking and weight training increased the PA levels of sedentary adolescents. Theory-based motivational approaches are vital for influencing physical activity behavior. Programs that strengthen self-regulation of adolescents, such as intentions to be active and a sense of coherence, promote activity beyond school settings. Future research should establish a consistent definition of “sedentary” to enable better comparisons across studies. Sedentary adolescents should be identified as a separate group, and their perspectives and the design of the intervention should be considered in terms of intensity levels, support needs, goal setting,and progress tracking. Sedentary adolescents need engaged leaders who can provide individualized support and guidance, ideally within programs tailored to their needs. A combination of measures, self-reports, and objective measures of fitness should be used, preferably easily applicable measures.