In education, among the most anticipated consequences of the COVID-19 pandemic are that student performance will stagnate or decline and that existing inequities will increase. Although some studies suggest a decline in student performance and widening learning gaps, the picture is less clear than expected. In this study, we add to the existing literature on the effects of the COVID-19 pandemic on student achievement. Specifically, we provide an analysis of the short- and mid-term effects of the pandemic on second grade reading performance in Germany using longitudinal assessments from over 19,500 students with eight measurement points in each school year. Interestingly, the effects of the pandemic established over time. Students in the first pandemic cohort even outperformed students from the pre-pandemic cohorts and showed a tendency towards decreased variances during the first lockdown. The second pandemic cohort showed no systematic mean differences, but generally had larger interindividual differences as compared to the pre-pandemic cohorts. While the gender achievement gap seemed unaffected by the pandemic, the gap between students with and without a migration background widened over time--though even before the pandemic. These results underline the importance of considering effects of the pandemic across cohorts, large samples, and fine-grained assessments. We discuss our findings considering the context-specific educational challenges and in terms of practical implications for teachers' professional development.