Schoolwide positive behavior support (SWPBS) is a systems-level intervention designed to prevent the occurrence of problem behavior and increase social competence. A growing body of research documents that SWPBS reduces problem behavior and improves academics (e.g., McIntosh, Chard, Boland, & Horner, 2006), yet documentation of the feasibility of implementing SWPBS in high school settings is lacking. The current study examines implementation of universal SWPBS components in eight high schools serving over 15,525 students across a three-year period. Our findings were that improvements in implementation were evident between baseline and the end of year one, yet the implementation of SWPBS practices took a minimum of two years to achieve statistically significant and meaningful changes. These results suggest that unique aspects of the high school context may present specific implementation challenges. (Contains 4 tables and 1 figure.)