Changes in adolescents’ reports of social support from parents and friends were examined across the 4 years of high school to examine associations between support and academic achievement. Results from 541 adolescents from diverse backgrounds suggest that changes in encouragement from parents and friends within individual adolescents are associated with concurrent changes in grade point average (GPA). Between-person analyses indicate that adolescents who report higher levels of encouragement in ninth grade are more likely to enroll in courses and receive grades that make them eligible to enroll in California's public university system. Both GPA and eligibility, in turn, were associated with higher rates of enrollment in college after high school. Many of these findings varied according to ethnicity and generation, reinforcing the importance of understanding the extent to which adolescents from diverse backgrounds have access to the information and support that is necessary to take advantage of the postsecondary educational system.