The quality of a school's social environment is critically related to student outcomes, including academic performance, attendance, student behavior, and high school completion rates. New Hampshire engaged in a dropout prevention initiative between 2006 and 2012 that focused on implementation of the multitiered Positive Behavioral Interventions and Supports (PBIS) framework combined with an intensive, student-driven school-to-adult life transition intervention for the highest-need youth. This article presents a case study of how one high school in New Hampshire implemented PBIS at all three tiers of support: schoolwide, targeted, and intensive. The case study includes a description of practices implemented by the school, school- and student-level outcomes pre- and postimplementation, the coaching and training support provided to school staff, and successes and challenges experienced by the school. The discussion ends with recommendations for practice and research of PBIS implementation in high schools.