Teaching mathematics, especially in urban elementary schools, is a complex endeavor requiring specialized, expert level knowledge of mathematics, pedagogy, and student development. Drawing from social cognitive theory (Bandura, 1986) and social capital theory (Leana & Pil, 2006), this study within the elementary schools in an urban New England school district explores the inter-relationships of mathematics knowledge for teaching, mathematics efficacy for teaching, professional support networks, and contextual factors, as elements found to influence instructional effectiveness and student mathematics achievement. Based on needs assessment data that indicated weak perceptions of collective mathematics efficacy for teaching, connected to a lack of collegial trust, an intervention was implemented to develop critical colleagueship among the district’s school-based, elementary mathematics coaches as sustainable support for collective mathematics efficacy for teaching and professional growth, through active engagement in collaborative inquiry teams within a blended learning environment. A mixed methods analysis of process and outcome data indicated statistically significant, positive changes to participants’ perceptions of collective mathematics efficacy for teaching and internal social capital. Findings indicate focusing future professional development efforts for school-based instructional leaders on promoting internal social capital may be an effective approach to promoting collaborative and sustainable professional learning, as well as students’ mathematics achievement
