Duffy was a high school principal for sixteen years, during which he served as President of Vermont Principals' Association and Chair of the New England Association of Schools and Colleges Commission on Public Secondary Schools. Since co-founding TLS in 2002 Duffy has worked closely with large urban districts, suburban districts, small rural districts and state departments of education in the areas of teacher and administrator supervision and evaluation, coaching, embedded professional development, school reform, curriculum, instruction, and on-going formative assessment. Additionally, he has worked with Educational Testing Services (ETS) as a national trainer for Danielson’s, Framework for Teaching, and on various research projects.