The aims of the study were to verify the effect of an intervention on the stage of behavior change for TV viewing and to identify the possible mediating role of knowledge on screen time guidelines among Brazilian students. This study is a multicomponent school-based intervention conducted with 727 students (54.3% female, 13 years; 427 in the intervention group and 300 in the control group). The intervention was performed in 2017 with baseline and post-intervention assessments (over one academic year). Teacher training was carried out at the beginning of the intervention along with environmental improvements, while educational curriculum such as the delivery of folders and posters were performed throughout the year. The stages of behavior change for TV viewing and knowledge of screen time guidelines were measured by a self-reported questionnaire and the structural equation modeling approach was performed. The intervention had no significant effects on stages of behavior change for TV (p = 0.744) nor did it result in significant changes on knowledge of screen time guidelines (p = 0.741). While there was no mediation between knowledge of screen time guidelines and the effect of the intervention on stages of behavior change for TV (95%CI: -0.45;0.63), an association was found between knowledge of screen time guidelines and stages of behavior change for TV (p<0.001). In conclusion, intervention had no significant effect on the stages of behavior change for TV and no mediating effect was observed. However, enhancing awareness on screen time guidelines may positively impact the adolescent’s intention to reduce TV time.