Background: Federal and state accountability policies attempt to improve educational outcomes but have been blamed for a breadth of ills, including minimizing local knowledge and reducing teachers' ability to respond to contextual needs. Teachers in high-needs schools, especially, feel the effects of constrained curricula and increased testing, resulting in increased workload and anxiety. Purpose: This article explores the impact of professional development on teachers and students in a time of high-stakes accountability. Specifically, we ask: Does Lesson Study impact teachers' instruction and students' achievement in writing? And how do pressures imposed by policy impact efficacy and collaboration in a high-needs school? Research Design: This study uses data from 20 Lesson Study meetings at a high-needs, "Turnaround" school and considers changes in students' writing achievement. The mixed-method approach and high-stakes context offer a unique contribution to Lesson Study research. Conclusions: Findings indicate that instruction changed and students' writing significantly improved, with the mean growth percentile increasing from the 30th to the 46th percentile on state assessments. Further, we found that during the Lesson Study process, teachers moved through six Stages of Transformation in response to a high-pressure context, moving from feelings of anger and blame-shifting to eventual feelings of empowerment.