Social and emotional learning (SEL) programs have become increasingly popular during the last 20 years. Although the benefits of SEL programs are significant, the issue of implementation fidelity often arises. The purpose of this study was to assess the impact of implementation fidelity in the Life Skills Training (LST) program implemented with middle school students in a large Florida school district. Three core elements of implementation fidelity were assessed including: (1) adherence; (2) participant responsiveness; and (3) quality of delivery. Student survey data were collected from 4812 students and 104 classrooms in 16 middle schools that participated in the LST program. Multilevel modeling was used to assess the effect of individual-level [gender, race/ethnicity, and socioeconomic status (SES)] and classroom-level characteristics (adherence, participant responsiveness, and quality of delivery) on students' SEL outcomes measured at posttest. At the individual level, results indicated that students' race/ethnicity and SES were significantly associated with predicting student SEL outcomes at posttest. At the classroom level, participant responsiveness was significantly associated with predicting student SEL skills at posttest. Findings are discussed in terms of implications for research and practice.