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Although in-service teacher training programs are designed to enhance the performance of several cohorts of students, there is little evidence on the persistence of their effects. We present the two-year results of a randomized study of an intensive in-service teacher training program conducted in France during and after the training program’s implementation. Our results highlight the short-run effectiveness of the training program: it successfully improves students’ performance but only during the implementation year. A detailed analysis of teachers’ outcomes indicates that teachers changed their pedagogical vision and practices but afterward struggled to apply skills to contents not directly covered during training.