Through a sociolinguistic analysis of interactional spaces created by the teachers and students at one 50/50 dual immersion school, this study examines how teachers present and implement this school's language policy and how these practices are reproduced and transformed through the language choices of the students in their daily interactions. The analysis shows that strict enforcement of the instructional separation of the two languages may emphasise a division of interactional spaces and language groups where only Spanish or English is used. The findings illustrate how the teachers and certain children are becoming marked as speakers of either Spanish or English and how their thickening identities may consequently limit the opportunities to socially interact in and practice the second language at this school site. Unless focused efforts are assigned to the creation of interactional spaces where both English and Spanish are used interchangeably by the same speaker for various purposes, we argue that simply "learning in two languages" may not necessarily lead to interactional spaces that foster the development of bilingual speakers.