This study tests the hypothesis that teacher-led collaborative inquiry cycles, guided by instructional standards, lead to improved teacher performance and effectiveness. We examine the impact of teachers' self-selection into teacher peer excellence groups (TPEGs), which involves lesson co-planning, peer observation and feedback, and collaborative lesson-plan revision on participating teachers from 14 pilot public schools in Tennessee. Using survey results and statewide administrative data, we apply a propensity score matching strategy, and find that TPEG teachers experience growth in their instruction ratings and value-added scores in the subsequent year, although the longer term impact is attenuated. We contribute to the literature by identifying deprivatized practice and instruction-focused collaboration as key features of teacher communities of practice, highlighting the importance of using standards-based instructional quality measures, linking participation in collaborative inquiry cycles to teacher-level outcomes, and estimating effects applicable to situations in which teachers exercise agency and collaborate voluntarily.