Sociocultural studies of science education have consistently recognized the dialectic nature of students' agency to create and author positions for themselves and the structural constraints that may influence them. This mixed-methods study explores one particular aspect of these potential constraints: the possibility of a social structure specific to school science, using the concept of a science student role as an indicator. The first phase of the study was qualitative and exploratory, using open-ended questionnaires and interviews to understand students' views of the expectations placed on them as science students (n = 95). The second phase was quantitative (using items developed from the qualitative analysis), with both exploratory and confirmatory elements (n = 157). Results suggest clear and explicit role understandings among these students, characterized by references to expectations of intelligence, experimental skill, scientific mindedness, and appropriate classroom behavior. The consistency of these expectations across genders, science teachers and schools provides evidence that there is an element of social structure specific to school science that needs to be considered in studies of student agency and identity. (Contains 4 figures and 6 tables.)