Schools are increasingly adopting universal social-emotional learning (SEL) programs to support students' prosocial development and academic success. When adopted across contexts and student populations, SEL interventions can be implemented in different ways, particularly under typical classroom conditions that are not part of research efficacy trials. This study, situated across 13 elementary schools, examined 41 primary teachers' use of a popular universal SEL program with their 811 students, with attention to the prevalence and nature of teachers' program changes to standard program practices. In addition, this study explored whether and how teachers' changes were associated with instructional quality more broadly. Results from 221 lesson observations revealed that teachers' instructional expertise in areas closely aligned with the program's target intervention skills was positively associated with higher levels of program fidelity. Expertise was also related to program changes that honored students' outside of school experiences, supported moment-to-moment decision-making, and centered on students' interests.