This study explores the effect of school reopenings during the COVID-19 pandemic on married women's labor supply. We proxy for in-person attendance at US K-12 schools using smartphone data from Safegraph and measure female employment, hours, and remote work using the Current Population Survey. Difference-in-differences estimates show that K-12 reopenings are associated with significant increases in employment and hours among married women with school-aged children, with no measurable effects on labor supply in comparison groups. Employment effects of school reopenings are concentrated among mothers of older school-aged children, while remote work may mitigate effects for mothers of younger children.