Purpose: In this study, we investigated the effects of a schoolwide program, Bookworms K-5 Reading and Writing, on student achievement. Method: The study included seven cohorts of students (N = 8,806) in grades 2-5 in 17 elementary schools across three school years. We used a comparative interrupted time-series design, conducting multilevel growth curve models of Measures of Academic Progress reading scores with up to 10 data points per student. By modeling each student's growth curve, including a time by treatment interaction term, we were able to estimate the change in students' achievement trajectories corresponding to the implementation of Bookworms. Results: Results confirm a significant positive impact of Bookworms on achievement, with gains compounding over time and producing an overall standardized effect size of 0.26 by the end of 5th grade. Students who began third grade with relatively weaker achievement experienced more growth than those with average achievement, and those with average achievement experienced more growth than those with the highest achievement. Conclusion: This study provides evidence that a comprehensive literacy curriculum that emphasizes high-volume reading of grade-level texts and the use of evidence-based instructional practices produces positive effects on student achievement for students with a range of initial reading achievement.