Teachers' engagement in educational reform is an emotional process, yet the emotional dimensions of school improvement efforts remain understudied. This paper explores the emotional experiences of two teacher teams navigating a reform centering collaboration for instructional improvement. Drawing on group emotion (Kelly and Barsade in Org Behav Hum Decis Process 86(1):99-130, 2001) as a conceptual framework, this paper illustrates how individual beliefs and dispositions, group norms, and broader contextual factors mediate group emotion and engagement in reform over time. Findings indicate that leaders should attend to how teacher turnover impacts group emotion and engagement in capacity-building, particularly when teachers lose team members who have previously driven productive collaborative planning. Findings also highlight the nested nature of grade-level collaboration, revealing the influential role of department norms and relationships.