This study evaluated the long-term impact of schoolwide positive behavioural interventions and supports (PBIS) on student academic achievement. In this quasi-experimental study, academic achievement data were collected over 9 years. The 21 elementary, middle, and high schools that achieved moderate to high fidelity to the Save & Civil Schools' PBIS model were matched with 28 control schools to assess academic gains. There were 5 years of baseline data (no intervention in treatment schools) and 4 years of intervention data, including 1 year of maintenance. Results indicate that implementation of the PBIS programme was significantly associated with increased student academic achievement (p = 0.001) and that the rate of change for students' academic achievement in treatment schools was greater than for students in control schools. This study suggests that PBIS programmes, such as Safe & Civil Schools' "Foundations," may complement other efforts to improve academic outcomes.