This article describes an educational design research program situated within a professional development school that led to the development of the Cultural Proficiency Continuum Q-Sort (CPCQ). The CPCQ is a tool that enables teacher educators to systematically examine preservice teachers' cultural competence concerning students who are minoritized, marginalized, and otherized within PreK-12 schools. The author provides background and rationales for the need of the CPCQ together with a discussion of the educational design research program that facilitated the design of the CPCQ. Findings for this study are discussed discursively addressing the following question: (a) What are the rationales and chain of decisions that warranted advancing the development of the CPCQ through each phase of an educational design research program? and (b) What are participants' perceptions of the emerging tool's effectiveness for facilitating critical self-reflection, inquiry, and dialogue concerning students who are minoritized, marginalized, and othered within PreK-12 schools? This study demonstrated that educational design research together with professional development schools are an ideal context to develop tools in a real-world setting aimed to address issues around racial and social justice and cultural competence within teacher education programs and PreK-12 schools and classrooms.