The purpose of the study was to better understand the extent to which middle school principals' transformational leadership styles affect teachers' data-informed instruction, the influence of teachers' data-informed instruction on middle school student achievement, and the extent to which transformational leaders affect student achievement through data-informed instruction. Therefore, for this study, survey data were collected using Kenneth Leithwood's Transformational School Leadership Survey, as well as Jingping Sun's Data-Informed Instruction Survey. Additionally, this study used Scantron performance data for sixth-grade math to determine the effects of transformational school leaders on student achievement through data-informed instruction. The results showed that transformational school leadership has no effect on middle school teachers' data-informed instruction, and middle school teachers' data-informed instruction has no significant effect on student achievement, with socioeconomic status controlled. The study also showed that transformational school leadership has no statistically significant effect on student achievement through teachers' data-informed instruction. The results of this study help us understand the extent to which transformational school leadership affects student achievement through teachers' data-informed instruction. It also provides knowledge that will help us better educate aspiring leaders, making them effective transformational leaders in schools.