The COVID-19 pandemic has been a seismic and on-going 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 two years of the pandemic. Average fall 2021 math test scores in grades 3-8 were 0.20-27 standard deviations (SDs) lower relative to same-grade peers in fall 2019, while reading test scores decreased by 0.09-0.18 SDs. Achievement gaps between students in low-poverty and high-poverty elementary schools grew by 0.10-0.20 SDs, primarily during the 2020-21 school year. Observed declines are more substantial than during other recent school disruptions, such as those due to natural disasters.