This paper uses a multilevel analysis of mathematical reasoning rooted in Cultural Historical Activity Theory to examine how mathematical discourse and student reasoning about linear functions developed across 3 weeks in a ninth grade bilingual algebra class. Despite the teacher's expertise teaching with a conceptual focus, and her stated intention to focus on slope as a rate of change, the case study students in her class appeared to appropriate a procedural understanding of slope. By examining the nested activity systems of the students' group discussions, the classroom, school, and district, this analysis shows how external assessment pressures shaped the teacher's selection of tasks, her and her students' use of mathematical discourse, and ultimately, her students' opportunity to learn a critical algebraic concept.