We investigate the impact of an ambitious provincial school reform in Canada on students. mathematical achievements. This reform provides advantages for the purpose of evaluation and cuts across some of the methodological difficulties of previous research. First, the reform was implemented in every school across the province in both primary and secondary schools. Second, we can differentiate impacts according to the number of years students are affected by the reform. Third, our data set provides a longer observation period than typically encountered in the literature. We find negative effects on students’ mathematical achievements at all points of the skills distribution.