Gender imbalance in time spent on child rearing causes gender inequalities in labor market outcomes, human capital accumulation, and economic mobility. We conduct a large-scale field experiment with a near-universe of US schools to investigate a potential source of this inequality: external demands for parental involvement. Schools receive an email from a fictitious two-parent household with a general inquiry and are asked to call one of the parents back. Mothers are 1.4 times more likely than fathers to be contacted. We decompose this inequality into discrimination stemming from differential beliefs about parents’ responsiveness versus other factors and demonstrate that the gender gap in external demands is associated with various measures of gender norms. We also show that signaling father's availability substantially changes the gender pattern of callbacks. Our findings underscore a process through which agents outside the household contribute to within-household gender inequalities.