I designed an explanatory sequential mixed-method study to explore the relationship between leadership practices, collective teacher efficacy (CTE), and educational outcomes of low SES students in an open school system. Four data sources were analyzed: K-5 student Developmental Reading Assessment (DRA) results of 1,170 students, Collective Teacher Belief Scale (CTBS) survey to measure CTE perceptions of 48 teachers, focus group interviews composed of a stratified sample of 11 K-5 teachers, and two one-on-one principal interviews. The study took place in two elementary schools in north Texas during the 2020-2021 school year. A Spearman's rank-order correlation analysis indicated that the relationship between CTBS scores and student reading scores was mixed. While one school showed a positive association between CTE and the reading data of low SES students, the other school showed a weak correlation between the variables. The quantitative data indicated that CTBS scores did not independently explain reading achievements at both campuses. The data also showed that while teachers had a large effect size on the reading performance of low SES students, as measured through a Cohen's d for paired sample t-test, achievement gaps continued to widen. Two themes emerged through a grounded theory approach when principals described their sensemaking and framing process: many variables and teamwork. In a complex open system, school principals must consider the many needs of teachers and students before enacting reform efforts. To achieve ambitious goals, school principals promote teamwork and help develop supportive structures to assist teachers and students. Furthermore, the theme of support emerged when teachers described their CTE perceptions concerning their principal's leadership actions. The overall data suggest that leadership practices directly impact CTE levels and indirectly impact student performance. When teachers felt supported, CTE levels increased, but when teachers felt overwhelmed or failed to see how school and district mandates support teaching and learning efforts, CTE levels dropped. [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.]