We start with the stance that it is important for educators to understand students’ language repertoires in relation to characteristically mathematical conceptualizations and processes. The data in our study of grade 3 to 11 students’ language repertoires for conjecture led our attention to competing discourses in the classroom and the consideration that the mathematical language repertoires are also repertoires for friendship, competition, etc. In this paper we use a framework for identifying authority structures in mathematics classrooms to focus on formative communication acts, and then we consider how each act might serve a purpose for positioning the students involved in terms of each discourse that we identify as in play.