This linguistic anthropological case study demonstrates the ways in which a heterogeneous lab group of adolescents in a ninth-grade physics classroom constructed "raciolinguistic ideologies" that intersected with their locally constructed notions of science expertise. Using ethnography and discourse analysis this study unpacks how students in one lab group, guided by their teacher's instruction, constructed a model of personhood for science expertise that largely excluded female and Latinx students. Mock Spanish emerged as a central practice through which this marginalization was achieved. Implications for promoting equity for multilingual youth in heterogeneous classrooms are discussed.