This case study explores policy implementation of a Science, Technology, Engineering, and Mathematics (STEM)-focused charter school over a three-year period. Gateway Science Academy of St. Louis (GSA), a K-12 district, has applied the STEM-focused curriculum model for seven years. This case study uses the Fullan Change Theory's seven core principles to investigate GSA district's implementation and integration of the New Generation Science Standards (NGSS) and Project Lead The Way (PLTW). The purpose of this study is to explore the GSA school district's STEM policy implementation within the lens of the Fullan Change Theory. The researcher interviewed teachers and administrators and conducted classroom observations in order to collect data to investigate policy implementation. The study also reviewed relevant literature on effective policy implementations. In addition to interviews and classroom observation data, additional data was collected and analyzed, including teacher lesson plans, standardized test scores, and monthly activity logs. Findings indicated that the most successful STEM-policy implementation depends on staff motivation, administrative support, professional development, and team-lesson planning.