The aim of this study was to understand the role of school satisfaction on life satisfaction, according to gender, age, body mass index (BMI), and physical activity (PA) level. This was a cross-sectional descriptive study, carried out on 2823 adolescents (1396 boys and 1427 girls), aged between 12 and 16. A specific questionnaire to measure life satisfaction (Brief Multidimensional Student Life Satisfaction Scale (BMSLSS)), a questionnaire to measure satisfaction with school (“Life circumstances of Young people: School”), and the International Physical Activity Questionnaire (IPAQ) were used to analyze PA practice. Gender, age, and BMI were used as control variables. In general, the main results showed that school satisfaction had a clear role in life satisfaction. Similarly, the findings allowed us to conclude that the role of school satisfaction on life satisfaction was more evident in male school children, those who were older, or those who have a higher BMI. The regular practice of PA enhanced school satisfaction and its role on life satisfaction. Therefore, it is very important to assess the importance of school satisfaction as a determinant of quality of life and the adoption of healthy habits, recognizing the fundamental role of teachers in this regard.