We evaluate the impact of a large-scale teacher coaching program, in a context of high teacher turnover, on teachers’ pedagogical skills. Previous studies find that small-scale coaching programs can improve teaching of reading and science in developing countries. However, scaling up can reduce programs’ effectiveness, and teacher turnover can erode compliance and cause spillovers onto non-program schools. We develop a framework that defines different treatment effects when teacher turnover is present, and explains which effects can be estimated. We evaluate a Peruvian teacher coaching program, exploiting random assignment of that program’s expansion to 3,795 rural schools in 2016. After two years, teachers assigned to the program increased their aggregate pedagogical skills by 0.20 standard deviations. The program also increased student learning, and the schools whose teachers’ pedagogical skills increase the most are also the schools with the highest increases in learning, indicating that pedagogical skills are one mechanism linking teacher coaching to student learning.