For much of the 20^th century, the dominant image of teachers in research on the elementary school workplace was that of isolates, practicing alone behind classroom doors. Over the closing decades of the 20^th century, however, another portrayal of the school workplace, one where teachers work together to plan lessons, solve instructional problems, and improve their teaching emerged in the literature. More specifically, scholars have shown how a school’s and/or an individual’s social capital can provide access to resources, such as information, that enable them to develop new knowledge and skills that in turn contribute to improvement in performance.  In this paper, the authors define the construct of educational infrastructure and use it to empirically examine and theorize how educational infrastructure, as designed and implemented by school systems and schools, structures school staff interactions about instruction – a necessary if not sufficient condition for the development of social capital.  By educational infrastructure the authors mean those resources that school systems and schools design and mobilize to guide classroom teaching, maintain teaching quality, and enable instructional improvement  (e.g., curricular materials; student assessments; procedures and routines for analyzing evidence of instructional processes and outcomes; professional learning opportunities; and procedures and routines for structuring the work, such as routines for hiring, mentoring, developing, and evaluating teachers).   Using longitudinal quantitative and qualitative data from all 14 elementary schools in one local public school system, we explore how the school system’s redesigned educational infrastructure (e.g., organizational routines, leadership positions, new curricula) influences school staff interactions about mathematics instruction.  Further, we consider how the school system’s formal educational infrastructure interacts with the school building’s physical infrastructure - propinquity -  to influence who school staff interact with about mathematics teaching and its improvement. Accordingly, our main research question is this: How—if at all—does propinquity in interaction with formal educational infrastructure  influence interactions among school staff about instruction? 

We take up our research question by bringing the literature on propinquity in organizations, mostly in organizations other than schools,  into dialogue with the literature on teacher interactions in the schoolhouse. Our study documents how propinquity–the elephant in the schoolhouse that has been largely ignored by prior work on school staff interactions about instruction–influences interactions among elementary school staff and does so in interaction with formal educational infrastructure arrangements. Using a mixed-methods research design, we first show that propinquity matters to interactions among school staff and then, using both qualitative and quantitative data, we theorize how it matters to these interactions. 

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