In this study, the authors tested the effects of Tekster [Texter], a comprehensive strategy-focused writing instruction program, using a switching replication design with three measurement occassions. The program was implemented by fourth, fifth, and sixth grade teachers (N = 76) in 60 general education classrooms in the Netherlands. Students (n = 688) and teachers (n = 31) in Group 1 worked with Tekster during the first 8-week period, between the first and second measurement occasion. Students (n = 732) and teachers (n = 45) in Group 2 implemented Tekster during the second 8-week period, between the second and third measurement occasion. The intervention led to statistically significant improvements in the quality of students’ writing. The effect size for the full sample was 0.32 and 0.40 for students who students who completed all 16 Tekster lessons. Gains shown by students in Group 1 were maintained after 8 weeks. Because writing quality was assessed in 3 genres, the findings are generalizable across students, classes, and writing tasks. Taken together, the results of this study demonstrate that a strategy-focused writing instruction program, such as Tekster, can be an effective way to improve upper-elementary students’ written language skills.