Principals are faced with daily challenges to meet changing expectations in a field that has become unpredictable and demanding. Additionally, it is the responsibility of the principal to develop, monitor, and maintain structures and conditions that best ensure both instructional and academic success. The purpose of this mixed-methods case study was to investigate middle school teachers' perceptions regarding principal practices, the principal reported implementing and the teachers acknowledged, as influencing collective teacher efficacy. Additionally, the purpose was to determine any discrepancies between principal reported implemented practices and teacher's recognition of implemented practices which contributed to influencing collective teacher efficacy. This mixed methodology case study investigation was conducted in one highly efficacious, high achieving middle school. In Phase I, a survey was administered to thirteen middle school teachers to collect quantitative data measuring levels of collective teacher efficacy. In Phase II, a survey was administered to the middle school principal to collect quantitative data for determining the implementation of principal practices related to collective efficacy. In Phase III, nine teachers from Phase I were interviewed using an interview protocol to collect qualitative data regarding teacher perceptions of implemented principal practices most influential to collective teacher efficacy. Teachers indicated high levels of collective teacher efficacy within the school. The principal reported high levels of implementation in leadership practices related to the functions of framing school goals, communicating school goals, supervising and evaluating instruction, and coordinating the curriculum. The major themes from qualitative interview data analysis supported the quantitative reports of implemented practices and further indicated which structures and practices most support collective efficacy development: supportive school culture, positive school climate, and systems of support. The explanation of these findings provided the following conclusions and ideas: (1) Developing a strong school culture is connected to a strong sense of collective efficacy; (2) Principals must establish equitable systems for efficacy development, even when efficacy levels are already high; and (3) Ongoing collection, monitoring, and assessment of triangulated data related to efficacy and principal practices will improve collective efficacy. These conclusions suggest educational leaders may generalize their own implementation of principal practices and levels of collective teacher efficacy and use the findings from this study as a resource for increasing efficacy at the individual and collective level. [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://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]