School choice policies seek to increase access to educational opportunities and stimulate innovations in schooling. This study examines the early stages of one such innovation--school-wide curricular specialization--in three Catholic elementary schools adopting a STEM focus and uses interviews to consider how and why different levels of support exist for the shift and under what conditions private and religious schools are prepared to make significant changes in instructional practice. Findings suggest that school resources--material, human, and social along with professional development--play an important role in shaping engagement in the adoption of a school-wide curricular focus.