This longitudinal study explores three research questions. First, what is the prevalence of math and science gender stereotypes among high school students, their parents, and teachers? Second, are parents' and teachers' gender stereotypes related to adolescents' stereotypes? And third, are adolescents' gender stereotypes associated with their math and science identity and outcomes? We used a nationally representative U.S. sample (N = 22,190, 50% girls, 53% White, 22% Latinx, 13% Black) of adolescents surveyed at 9th and 11th grade, their parents, and teachers. Adolescents' transcripts were also collected at the end of high school. Adolescent gender stereotypes became significantly more traditional from 9 to 11th grade. Parents were three times more likely to believe that males are better at math/science (compared to believing females are better), and we found significant positive relations between parents' and adolescents' stereotypes. Finally, adolescents' math/science gender stereotypes were significantly related to their math/science identity, which in turn was related to their STEM outcomes over the course of high school. Our findings give insight to the development of academic gender stereotypes in adolescence, their potential precursors, and their relations to academic outcomes.