Despite the popularity of teacher leadership since the 1980s, little research examines its effects on student achievement. In this paper, I assess the influence of the New York City Department of Education's Teacher Career Pathways program, a teacher leadership initiative, on student achievement in grades three through eight. Using difference-in-difference approaches, including new event study estimators, I find that where school leaders staffed teacher leaders into formal roles with defined responsibilities, positional authority, and commensurate salary increases, student achievement in ELA and math improves. Moreover, the improvement in scores compounds over time, with schools exhibiting increasing gains in each year following the initial introduction of teacher leaders. Schools that do not staff teacher leaders do not observe similar outcomes. I consider these results in the context of democratic policymaking and teacher empowerment, suggesting that teachers must be formally empowered in schools to lead meaningful changes that ultimately improve student achievement.