The COVID-19 pandemic has been a seismic and ongoing disruption to K-12 schooling. Using test scores from 5.4 million U.S. students in Grades 3-8, we tracked changes in math and reading achievement across the first 2 years of the pandemic. Average math test scores in the fall of 2021 in Grades 3-8 were 0.20-0.27 standard deviation (SD) lower relative to same-grade peers in the fall of 2019, while reading test scores decreased by 0.09-0.18 SD. Achievement gaps between students in low-poverty and high-poverty elementary schools grew by 0.10-0.20 SD, primarily during the 2020-2021 school year. Achievement disparities by student race/ethnicity also widened substantively. Observed declines are more substantial than during other recent school disruptions, such as those due to natural disasters.