This study aimed to investigate the contribution of teacher support and parental monitoring to academic performance over three years, testing the mediating role of self-determined motivation and academic self-efficacy and establishing whether the role of teachers and parents varies over time. A total of 419 adolescents--201 males (48%) and 218 females (52%), M[subscript age] = 14.34 years (SD = 0.90)--attending at T1 the ninth grade of schooling participated in the study. The questionnaires were administered three times over three years in February 2016 (T1), February 2017 (T2), and February 2018 (T3). Specifically, at T1, the students answered questionnaires regarding parental monitoring and teacher support. At T2 and T3, the students completed questionnaires about self-determined motivation and academic self-efficacy. At the end of each school year, in June 2017 (T2b) and June 2018 (T3b), the teachers' assessments of the students' academic performance expressed in school marks were collected. A structural equation model was used to test the hypothesised longitudinal relations between the study variables. The results showed that teacher support and parental monitoring directly and positively affected motivation and self-efficacy over time, which, in turn, impacted academic performance positively. The results also indicated that teacher support and parental monitoring indirectly affected academic performance over time through the mediation of motivation and self-efficacy and that the parents' influence was highest on motivation, while the teachers' influence was highest on self-efficacy. These results suggest the importance of implementing interventions aimed at enhancing parental monitoring and teacher support to improve students' academic performance.