In the United States, a concerning number of middle and high school students are yet to master fundamental reading skills. 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 in a large urban Southeast school district. The district uses Lexia® PowerUp Literacy® as part of its multi-tiered system of support. The study leverages the log-files and usage data from about 26,000 students exploiting differences in differences and event study estimations. The results indicate that, on average, PowerUp increases test scores by 0.14 SD. Also, the magnitude of the effect is larger for students who used PowerUp consistently and among English Language Learners (0.25 SD). 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 to narrow the achievement gap among students, particularly for English Language Learners.