This longitudinal, multi-cohort study examined the first-year implementation of Positive Behavioral Interventions and Supports (PBIS) within a decade-long university–district partnership grounded in the Whole School, Whole Community, Whole Child (WSCC) framework. Using the Schoolwide Evaluation Tool, we assessed Tier 1 PBIS fidelity across three cohorts of schools and analyzed changes in student disciplinary incidents and chronic absenteeism. Schools demonstrated significant improvements in implementation fidelity from fall to spring (Cohen’s d = 0.79), with the largest gains observed in data-based decision-making and systems for responding to behavioral violations. Regression analyses revealed that higher Year 1 fidelity significantly predicted reductions in student disciplinary incidents (β = –0.34, p = 0.024), even after accounting for baseline performance, cohort, and school level. Although chronic absenteeism declined modestly, it was not significantly associated with fidelity. These findings address critical gaps in the literature by linking early implementation quality to student outcomes and highlighting the role of community-engaged partnerships in scaling evidence-based practices. Implications for practice include embedding data routines, aligning PBIS with WSCC supports, and building internal capacity to sustain districtwide improvements.