Background: Failure to complete secondary education often results from a process of educational disengagement. Studies of teen childbearing and high school completion have not adequately accounted for the role of school disengagement prior to conception and may overestimate causal impacts of teen childbearing. Methods: We link New York City birth and school records to study a cohort of 22,484 Black and Latina public school students. We measure disengagement with monthly absences from age 12 until the month before conception and identify five preconception attendance trajectories using group-based trajectory modeling. Results: Preconception absenteeism can account for as much as half the deficit in high school completion associated with a teen birth. This finding is robust to statistical approaches, such as school fixed effects and inverse-probability-weighted regression adjustment. Conclusions: Our results suggest that programs intending to increase educational attainment among young women should emphasize attendance and engagement, rather than primarily teenage pregnancy prevention.