Teacher attrition has been an issue that has burdened school leaders for decades. The rate at which teachers leave schools is costly and has a negative impact on student achievement. The current study examined the costs associated with teacher attrition and the impact on student achievement. In this mixed methods study, both quantitative and qualitative analyses were performed to better understand the impact that teacher turnover has on schools. For the quantitative portion of the study, the researcher studied the relationship between school district retention rates and achievement on state assessments and found a positive correlation existed. For the next phase, the researcher studied the potential relationship between the amount of money spent replacing teachers and the district's accountability rating. A correlation was not found in phase two. In the qualitative portion of the study, the researcher interviewed 12 principals to collected their perceptions related to the unseen costs of teacher turnover. The major themes that emerged were related to time and the impact on professional development. This study was intended to provide valuable data that school leaders can use to formulate strategic plans aimed at retaining highly qualified teachers, thus improving student achievement. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]