This dissertation collects three independent but interrelated studies exploring how school intra-organizational mechanism may mediate the impact of external interventions on improving teaching and learning. This first study examines how high-quality professional development (PD) can promote the diffusion of effective teaching strategies among teachers through collaboration. Drawing on sociometric data from a larger experimental evaluation study, this study shows that teachers' participation in professional development is associated with providing more help to colleagues on instructional matters. Moreover, the influence of professional development on teachers' instructional practice in writing spreads through this network of helping interactions in ways that augment the direct effect of participating in professional development on their practice. These findings suggest that in addition to direct effects of professional development, there can be spillover effects of professional development through collegial interactions. Evidence presented in this study will potentially help policymakers develop high-quality PD programs and distribute PD participants within schools to promote all teachers' performance. The second study investigates the role of formal and informal leaders in supporting the diffusion of external reforms within schools. In the context of implementing reading policies as part of the No Child Left Behind (NCLB) 2001 legislation, this study aims to examine a) how formal and informal leaders promote instructional changes through professional interactions with teachers; and b) which types of instructional practices are most responsive to which types of leaders. I analyze longitudinal data concerning both professional interactions about teaching reading and instructional practices of teachers and leaders in nine K-8 schools in a single state. I find that formal leaders convey normative influence on general teaching practices such as setting standards, selecting materials, and assessing students, while informal leaders convey normative influence on specific pedagogical practices of teaching basic reading skills. Findings contribute to the theoretical and methodological development of both distributed leadership and policy implementation within schools. Moreover, this study suggests the importance and several strategies for developing a strong instructional leadership team that recognizes the complementary influences of formal and informal leaders. The purpose of the third paper is to investigate the potential of using multilevel item response theory to estimate the depth of teacher interactions under the ego-centric framework, defined as the propensity of endorsing collaborative relations with regard to mathematics instruction. Using empirical data from a larger study of Middle School Mathematics and the Institutional Setting of Teaching, beyond estimating item rareness parameters (fixed effects) and the depth of interaction for each tie (random effects), this study also develops methods to gauge item goodness-of-fit and information function to examine the quality of social network survey instruments under a multilevel framework. Finally, this study demonstrates the possibility to incorporate predictors in the measurement model to investigate Differential Item Functioning (DIF) and explanatory research questions. The methodological development in this paper significantly contributed to the growing popularity of using network studies to inform educational policy and practices by providing a psychometrically sound measure of teacher interaction on professional matters. [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/en-US/products/dissertations/individuals.shtml.]