The goal of this study was to examine IXL usage among high school students in California and its relation to academic performance in math and ELA, as measured by the Smarter Balanced Summative Assessment (SBA). At the high school level in California, the SBA is taken only by 11th- grade students each year. We analyzed students' math and ELA SBA performance, using 2019 as the pretest year and 2021 as the posttest year. The key finding is that IXL implementation improves student achievement. Eleventh-grade cohorts that used IXL Math or IXL ELA performed better on SBA Math and SBA ELA, respectively, than comparable cohorts that did not use IXL. Specifically, the proficiency rate was nearly five percentage points higher for IXL Math cohorts and more than ten percentage points higher for IXL ELA cohorts relative to cohorts not using IXL.