Science teachers mediate social and academic language in science classrooms through teacher talk and classroom discourse. In multilingual classrooms, ways home and international languages are deployed can affect conceptual learning of science. This study investigates, through Bakhtin's dialogic perspective, multilingual language practices and language deployment within Lebanese grade 8 science classrooms and their influences on students' conceptual understanding and meaning-making of science. Data came from private and public middle school science classrooms with different SES. At least eight science lessons were videotaped for analysis. Science teachers were interviewed and video-based student focus group interviews were conducted. A multi-level dialogic framework was used to analyse language practices and participants' meaning-making. Classroom interactions were analysed based on communicative approaches (authoritarian/dialogic), patterns of discourse, and emerging science knowledge types. Languages deployed by students and teachers were also examined. It was found that within lower SES contexts (public schools), teachers deployed home language more fluently for different purposes. In lower-middle SES private schools, teachers adamantly used English even at the expense of conceptual learning. Teachers and students' meaning-making of their language practices involved both ideological and instrumental aspects. We discuss how different forms of authoritarian discourse intersect to legitimise and reproduce certain inequities.