School tracking is the practice of separating students into tracks or classrooms based on their academic achievement. While school systems around the world use tracking, some track more heavily than others. Tracking is controversial, as low-income students are more likely than high-income students to be placed in low-achieving classrooms or tracks, resulting in greater socioeconomic disparities in educational attainment and ultimately in the labor market. This article discusses the author's recent paper investigating how reducing the degree to which students are tracked at an early age affects their marriage and fertility outcomes. Specifically, I evaluate the consequences of a French reform that delayed the separation of students into academic and vocational tracks from age 11 to 13 and replaced it with a less intensive system of grouping students into achievement-based classrooms. The results show that the decisions school systems make about when and how to track students can have long-term consequences for social and economic inequalities.