Preparing teachers to enhance students’ STEM education usually takes place via continuing professional (CPD) programmes. Many STEM-CPD programmes are criticised for short-term, information-dominated, and non-interdisciplinary approaches that lack critical consideration of learning communities, teaching contexts or STEM teaching is likely to take place in inclusive secondary schools/classrooms. We evaluate a UK-based, national, eight-year+ STEM-CPD programme targeting inclusive secondary schools, using a bootstrapped approach combining qualitative insights and quantitative comparisons. We found high levels of teachers’ STEM competence, desire for student inclusion and perceptions of student impact. Yet, shortcomings in the operations of teacher networks, lack of pedagogic/social pedagogic support and school-based disciplinary boundaries inhibited effects of the programme. Theoretical considerations were built into methods development and evaluation outcomes.