The primary research question that the study attempted to address was: What impact does the transformational leadership practice of school principals have on students' academic performance in the eyes of teachers? The research design used in the study was a convergent parallel mixed method. The methodology for gathering data was sound, utilizing an open ended semi-structured interview during the qualitative phase and a modified MLQ-based questionnaire during the quantitative phase. Student academic performance was the dependent variable in the study. In contrast, transformational leadership behaviors, including idealized influence (IF), inspirational motivation (IM), intellectual stimulation (IS), and individualized consideration (IC), were the independent variables. Purposive sampling was used to choose the subjects, guaranteeing a representative and diverse sample for the research. The study's target population comprised 5956 teachers and 95 public secondary schools in Ethiopia's Sidama region. An appropriate sample approach was employed to conduct the study, which involved 362 teachers from 24 public secondary schools. Twenty teachers participated in the qualitative portion of the study. The collected data was analyzed using both descriptive and inferential statistics. According to the study's findings, most teachers perceived and agreed that their school's principal adopt transformational leadership practices. The study also showed that, with an overall correlation value of r =.779 (p<.01), the principal's transformational leadership practices significantly and positively impact students' academic attainment. The linear multiple regression analysis revealed that changes in the principal's transformational leadership approach account for 61.4% of the variation in student academic performance.