The burgeoning work of translanguaging and bilingualism has much to offer adolescent learning spaces in order to provide bi/multilingual students more equitable opportunities to engage in disciplinary literacy at the high school level, particularly where there are many low-incidence languages. Drawing from critical theories in both literacy and language research, we conducted this three-year study in two U.S. high schools (grades 9-12) in order to promote language equity and literacy engagement for emergent bilinguals and heritage speakers. We provided an intensive year of graduate courses on language, literacy, and equity for 27 teachers from various disciplines and school roles. Through analyzing their coursework, observations of their classes, and follow-up surveys, we documented how their heteroglossic language ideologies were nurtured, how they enacted translingual disciplinary literacies, and what benefits they perceived from this instructional approach. The findings illustrate how schools might overcome previously unquestioned monoglossic standards and linguistically oppressive systems through a whole-school translingual disciplinary literacies approach. Providing nuanced descriptions of how teachers engaged in translingual disciplinary literacy in various disciplines, we make a case for constructivist disciplinary literacy teacher education grounded in heteroglossic ideologies. We also draw connections from language equity to literacy engagement, suggesting that a translingual disciplinary literacies approach is a necessary instructional innovation to effect change in high school learning spaces for bi/multilingual learners. Finally, as our field pursues language equity and literacy engagement, like the teachers in this study, we must also critically evaluate our own ideologies toward literacy and language.