The present study investigates how perceived support from peers, parents, and teachers influences later academic performance and if academic self-efficacy and entity intelligence beliefs mediate this association in a sample of early secondary school students. Data were collected from 750 Norwegian students in lower secondary school at two-time points (8th and 10th grade). All support variables were positively associated with academic self-efficacy but not entity intelligence beliefs. Academic self-efficacy was positively associated with GPA in 10th grade, while entity intelligence beliefs were negatively associated. The only mediation effect observed was between teacher support and GPA through academic self-efficacy. The study offers knowledge about mechanisms of support and later GPA, emphasizing the vital role of teachers in promoting academic self-efficacy and, in turn, improving academic performance for young adolescents. Entity intelligence beliefs hamper GPA, but more research is needed to ascertain its effect on academic performance.