In this study we investigate the relationship between student achievement and a crucial aspect of teaching: curriculum implementation strategies. More specifically, we consider three strategies representing teachers' approach in dealing with heterogeneous classes: i) spending time on the same topic until everyone understands, ii) moving on to another topic even if part of the class does not understand the previous one, and iii) spending time to revise concepts and topics already studied in the previous year. We exploit the within-student between-subjects variation in the frequency with which different teachers adopt each of the three strategies to control for constant student and class traits and for the possibility that teachers may adapt their strategies to class composition. Our findings show that spending time on the same topic until everyone understands is not associated with a better performance of less able students. On the contrary, it produces substantial achievement losses for the most able ones. Spending time revising topics studied in the previous year increases the achievement of less able students without lowering the performance of the most able ones.