Background: Early sexual activity can undermine adolescents' future school success and health outcomes. The purpose of this study was to assess the role of a family homework component of a comprehensive sex education intervention in delaying sexual initiation for early adolescents and to explore what social and contextual factors prevent adolescents from completing these family homework activities. Methods: This mixed methods study included 6th-and 7th-grade survey responses from 706 students at 11 middle school schools receiving a sex education intervention, as well as interviews from a subset of 33, 7th-grade students from the larger sample. Results: Adolescents who completed more family homework assignments were less likely to have vaginal intercourse in 7th grade than those who completed fewer assignments, after controlling for self-reports of having had vaginal intercourse in 6th grade and demographic variables. Participants' explanations for not completing assignments included personal, curriculum, and family-based reasons. Conclusions: Family homework activities designed to increase family communication about sexual issues can delay sex among early adolescents and contribute to school-based sex education programs. Successful sex education programs must identify and address barriers to family homework completion.