Between 1850 and 1910, the share of young Americans living in towns with high schools increased from 17% to 46% - the fastest expansion of school access in U.S. history. Using new data on every high school in the United States, we show that this expansion transformed economic opportunities for many young adults but widened class and racial inequalities. We find sharp increases in school attendance rates for high school-aged children in towns that opened a high school relative to children in nearby towns without one. Linking children to adult outcomes, we show that high schools increased women’s labor force participation and job quality, while reducing the probability of early marriage and childbearing. Increased access to high school accounts for a third of the increase in women’s labor force participation between 1870 and1930. High schools had the largest effects on children from already-wealthy families, and did not, on average, benefit Black children. While the high school movement substantially narrowed gender gaps in labor market outcomes, it also widened existing race- and class-based disparities.