In this study we examined the impact of a year long model of professional development comprised of a monthly cycle of video-based self-reflection, peer coaching, and mentoring and bimonthly workshops focused on selected Classroom Assessment Scoring System (CLASS) dimensions. Education supervisors were trained and supported by project staff to lead coaching sessions with a team of teachers. Monthly changes in the quality of teacher-child interactions as measured by CLASS were examined. The intervention group (n = 38) was significantly different than a comparison group (n = 22) at the end of the year. There were significant increases in four dimensions related to behavior management, productivity, language modeling and quality of feedback. Similar patterns of change were found for teachers with and without college degrees. Effective instructional practices can be developed and implemented by teachers when they are provided multiple opportunities to engage in sustained professional development experiences based on a valid observational measure. Implications for designing cost effective CLASS-based professional development as well as limitations of this study are discussed. (author abstract)