Bilingual classrooms most often have strict language arrangements about when and who should speak what language to whom. This practice responds to diglossic arrangements and models of bilingualism developed in the 20th century. However, in the 21st century, heteroglossic bilingual conceptualizations are needed in which the complex discursive practices of multilingual students, their translanguagings, are used in sense-making and in tending to the singularities in the pluralities that make up multilingual classrooms today. Examining the case of a network of U.S. secondary schools for newcomer immigrants, the International High Schools, this article looks at how students' plurilingual abilities are built through seven principles that support dynamic plurilingual practices in instruction--heterogeneity, collaboration, learner-centeredness, language and content integration, language use from students up, experiential learning, and local autonomy and responsibility. As a result, students become not only more knowledgeable and academically successful but also more confident users of academic English, better at translanguaging, and more plurilingual-proficient. The article presents translanguaging in education as the constant adaptation of linguistic resources in the service of meaning-making and in tending to the singularities in the pluralities that make up multilingual classrooms today.