Youth in foster care face many academic risks, including diminished engagement in school and behavior problems that further threaten learning and academic achievement. However, school engagement is responsive to contextual change, leaving many opportunities to intervene upon school engagement and boost overall functioning. The present study examines the mediating and moderating potential of aspects within the relational context of school engagement and academically threatening behaviors, using a national sample of adolescent foster youth. Findings indicate that greater school engagement and more positive perceptions of relationships with both peers and foster parents were associated with lower self-reported internalizing, and lower self- and caregiver-reported externalizing behaviors. Peer relationships moderated the association between school engagement and youth-reported externalizing, and foster parent-youth relationships moderated the association between school engagement and youth-reported internalizing. Furthermore, peer relationships mediated the association between school engagement and internalizing, but not externalizing behaviors, while relationships with foster parents mediated associations between school engagement and both self- and caregiver reports of both internalizing and externalizing. Implications for future research and practice are discussed.