Declining enrolments in senior secondary science have heightened concerns for meeting the demands for more STEM-qualified workers and a scientifically literate society. Students' attitudes to science are formed during schooling, particularly in adolescence when they are exposed to a variety of science topics. Students' perceptions of their ability in science and their subjective task values are well established as predictors of their likelihood of engaging with and continuing their study of science. However, the role of classroom-based social support in supporting ability perceptions and task values is less well understood. In this study, we examined relationships between adolescents' perceived classroom-based social support, task values, and self-efficacy, and how these perceptions and attitudes predicted adolescents' intentions to study the three major science subjects (biology, chemistry, and physics) in senior high school. Participants were 475 adolescents in Grades 8 to 10 recruited from six schools in Sydney, Australia. Structural equation modelling was employed to test the hypothesised model in which social support from science teachers and peers predicted intended science subject selections through science self-efficacy, intrinsic valuing of science, and utility value of science. Results indicate that science teacher and peer support were not directly related to adolescents' intentions to study senior science subjects. Instead, they were indirectly related via their positive relationship with science self-efficacy and task values. Utility value was the strongest predictor of adolescents' intentions to study biology, chemistry, and physics, while self-efficacy and intrinsic value also predicted adolescents' intentions to study chemistry. These results suggest that classroom-based social supports are important for supporting adolescents' attitudes towards science, and that science utility value interventions may be useful in efforts to improve enrolments in senior science subjects.