The development of inclusive STEM high schools that have no academic admission requirements has been a national goal in the United States. However, there is no umbrella organization that gives guidance for structuring such schools. The purpose of this study was to develop and validate a self-assessment using critical components of successful inclusive STEM high schools for school personnel and educational researchers who wish to better understand their STEM programs and identify areas of strength. A multi-phase methodology was employed. In the first step, eight in-depth case studies were generated, and common themes were identified. In the second step, inventory items were iteratively generated and tested for reliability and validity, using a sample of 78 teachers and researchers in five schools. In the third step, the inventory was field-tested in a single school. The OSPRI inventory demonstrates strong content validity and reliability. Teachers and researchers generally responded similarly, although some differences emerged that are indicative of their respective experiences and perspectives. School stakeholders and educational researchers can use this inventory, in whole or in part, to better understand their STEM programs and establish a future agenda that best capitalizes on their strengths.