While teachers have long been acknowledged as a key part of educational innovation, research has largely neglected the impact that their buy-in has on reform initiatives. This study addresses this problem by examining the association between teacher buy-in and student academic growth under school reform programs, with a particular focus on how that relationship changes over time. Using multiyear data from the Study of Instructional Improvement, we arrived at two important findings. Counterintuitively, we found that teacher buy-in is negatively associated with student achievement in general. However, after controlling for program age, we also found that, in schools with more mature reform programs, higher teacher buy-in had a significant positive relationship with students' academic growth in all sampled subject areas. These findings provide useful insights into school reform policies, as well as important empirical evidence on the actual role of teacher buy-in in students' achievement.