A sequential exploratory mixed methods approach guided research on the science program infrastructures (SPI) of a sample of 28 high schools representing 1,370 high schools in a large southwestern state. Comparisons of SPI were made between and among schools representing highly successful schools of low (n=9) and high diversity (n=10) with less successful, high-diversity high schools (n=9). Content analyses of interview data from science program teacher liaisons revealed the following characteristics for typical high school SPIs: (1) shared leadership within a diverse community of actors, including teachers, school principals, special education teachers, department heads, content-area leaders, curriculum directors, and district representatives; (2) supports for shared, balanced decision making, including frequent meetings, established communication channels, and explicit department head responsibilities; and (3) actions extending beyond general tasks of disseminating information to include resolution of issues regarding state-mandated test scores, curriculum-related tasks, and choice of professional development topics. We include a discussion of the implications for including SPI as an important mediating layer within the ecology of high schools. The science program layer links administration with classrooms by employing efficient, equitable, and effective practices to advance the goals of science achievement and college readiness established at national and state levels for all high school graduates.