What happens during adolescence emerges from early in life and sets the stage for later in life. This linking function of adolescence within the life course is grounded in social, psychological, and biological development and is fundamental to the intergenerational transmission of societal inequalities. This article explores this life course phenomenon by focusing on how the social ups and downs of secondary school shape adolescents’ educational trajectories, translating their backgrounds into their futures through the interplay of their personal agency with the constraints imposed by the stratified institutions they navigate. Illustrative examples include gender differences in risky behavior, racialized experiences of school discipline, immigrant youths’ family relations, LGBTQ students’ school safety, STEM education, adverse childhood experiences, and mindset interventions.