In 2009, School-Wide Positive Behavioral Interventions and Supports (SWPBIS) was introduced in the Netherlands to support schools in creating safe learning environments. In this longitudinal study, we explored effects of SWPBIS on student outcomes in the Netherlands. Fidelity of implementation of SWPBIS has been associated with improved student outcomes. The purpose of this study was to examine the relation between changes in fidelity and student outcomes. A total of 66 elementary schools (n = 14,256 students) were followed for 3 years (2015-2018). We collected yearly data on fidelity, social safety (consisting of students' social well-being, general feeling of safety, harassment, prevalence of unsafe locations in and around schools), behavior incidents, and additional behavioral support. Using repeated measures analysis of variances, we saw an increase in fidelity scores and a decline in the percentage of students stating there were unsafe locations in and around school. Multiple regression analyses showed that changes in fidelity were related to changes in both students' social well-being and the number of behavior incidents. Limitations were discussed, such as the absence of comparison schools not implementing SWPBIS, and schools at different stages of implementation, and we accounted for missing data.