Pre-print, Working paper: Adapting instruction to the specific needs of each student is a promising strategy to improve overall academic achievement. In this article, I study the impact of an intensive in-service teacher training program on reading skills offered to kindergarten teachers in France. The program modifies the lesson content and encourages teachers to adapt instruction to student needs by dividing the class according to initial achievement. While assessing impact is usually difficult due to the presence of ability bias and teacher selection, I show that in this context, a value-added model that controls for school and teacher characteristics constitutes a legitimate strategy to estimate the treatment effect. Results show that all students benefiting from the program progressed in reading skills at the end of the year. Besides, weaker students progressed faster on less-advanced competences (such as letter recognition), while stronger students improved their reading skills. This suggests that teachers adjusted content to students' needs. Finally, a cost-effectiveness analysis reveals that the program is approximately three times more cost-effective than reducing class size in France.