The PAX Good Behavior Game (PAX-GBG) is an evidence-based universal preventive intervention program for classroom use. Our aim was to explore teachers' perceptions of PAX-GBG and their work with it, and whether this changed during the implementation of the intervention. In addition, we explored teachers' perceptions, and changes in perceptions, regarding how they perceived their workload when learning and using PAX, how it influenced relationships in the classroom and students with special needs. Semi-structured telephone interviews were conducted with six female teachers at three time-points during a school year. Based on thematic analysis, four themes illustrating teachers' experiences and the perceived effects of PAX-GBG on classroom relationships were identified: (1) Working with PAX-GBG; (2) The Game; (3) Focus on Behaviour; and (4) Relationships. A notable finding was that most teachers, at the outset, had concerns regarding the PAX Game and that these concerns mostly disappeared after they had experienced working with it. Based on improved student behaviours and overall enhanced relationships in the classroom, our results show that PAX-GBG is a suitable intervention for all students. Students with special needs may especially benefit due to a focus on clear expectations, positive reinforcement, and a more inclusive classroom climate. Some remaining concerns regarding the universal suitability of the game need to be explored further in future studies. Based on the results in this study, PAX-GBG seems to have high social validity in a Swedish context.