School closures due to the COVID-19 pandemic raised concerns about increases in educational inequality. We examined the magnitude of the impact of the first school closure for vulnerable student groups in particular. This study was conducted among 886 Grade 3-5 students in the Netherlands in schools serving a high percentage of students from disadvantaged backgrounds. Piecewise growth analyses indicated that the school closures caused discontinuity in students' achievement growth on national standardized tests and led to an average learning loss of 2.47 months in mathematics and 2.35 in reading comprehension, exceeding the duration of the school closure. Findings suggest that school closures contribute to educational inequality and indicate which students may particularly need additional support to overcome the adverse consequences of the lockdowns.