A concerning number of middle and high school students lack fundamental reading skills in the United States. One common way schools address this issue is by supporting those students with computer-assisted instruction. This study evaluates the causal effect of one such computer-assisted instruction intervention on English Language Arts achievement for middle and high school students in a large urban Southeast school district. The district uses a computer-based online learning platform as part of its multi-tiered system of support. The study benefits the usage data in the learning platform from about ten thousand students by exploiting difference in differences and event study estimations. Particularly, it offers a novel method by utilizing the time of initial platform usage and dates of within-year tests for each student. Our results indicate that, on average, the intervention increases test scores by 0.14 SD--a modest but important magnitude given the scale of the intervention. The magnitude of the effect is relatively larger for students who use the platform consistently and among English Language learners. Results are robust against several sensitivity tests including inverse probability weighting, and type of aggregated treatment effect parameter. These results suggest that effective computer-assisted instruction can help schools narrow the achievement gap among students, particularly for English Language learners.