Central to this dissertation was an examination of the role teachers' social networks play in schools as living organizations through three studies. The first study investigated the impact of teachers' social networks on teaching practices. Recent evidence suggests that teachers' social networks have a significant effect on teachers' norms, teachers' learning in communities of practice, distributed leadership, the implementation of innovations, and students' attainment including student learning and academic achievement in core subjects (Bryk & Schneider, 2002; Coburn & Russell, 2008; Frank et al., 2011; Spillane, 2006; Supovitz, Sirinides & May, 2010). Relatively little research, however, has been carried out on estimating the effect of teachers' social networks on teaching practices. The results of the first study indicated that the formal organizational structure of the school and teachers' social network structure at time 1 affect teachers' social networks at time 2, which affect teachers' teaching practices at time 3. In conclusion, the first study shows that teachers' social networks can improve teaching practice by changing formal (grade) and informal (subgroup) structure.The second study explored the effect of teachers' social networks on class composition. Previous studies show that class composition and peer effects have an important impact on students' learning (Burns & Mason, 2002; Dreeben & Barr, 1988; Harris, 2010). Methodologically, Value-Added Models have often been used to estimate the teachers' effects on student academic achievement with the assumption of random sampling and random assignment. Although there were studies about teachers' assignment between schools (Lankford, Loeb & Wyckoff, 2002) and students' assignment within schools (Rothstein, 2008), fewer studies have attempted to explore the effects of teachers' social networks on class composition. The results of the second study indicated that teachers' social networks affected class composition through non-random assignment of students to teachers with respect to students' previous academic achievement as well as economic status. Thus, the second research shows that teachers' social networks can indirectly affect students' learning by influencing class composition with respect to previous academic achievement as well as economic status. Finally, in the third study, I quantify the robustness of the statistical inferences models in chapters 2 and 3 for valid causal inference. Generally, observational studies may have weak causal inference due to differences in unobserved preexisting conditions as well as time order of cause. By quantifying the impact threshold of a confounding variable, however, we can evaluate the sensitivity of causal claims to an unobserved confounding factor. Thus, this study evaluates Impact Threshold of a Confounding Variable (ITCV) to invalidate the causal inference in chapters 2 and 3.In spite of limitations including missing values, reliability and validity of network measurement, and a limited sample, these chapters offer significant insight into the role teachers' social networks play in schools as organizations. Through a systematic analysis of the influence of these networks on key aspects of the student experience, this dissertation highlights the importance of teachers' social networks for teacher behaviors and in learning contexts.
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