As the global economic competition gets tougher, American policymakers and researchers are interested in finding ways to increase the number of students pursuing STEM (Science, Technology, Engineering, and Mathematics)-related majors in order for the United States to continue its role as an economic powerhouse. A survey study was employed to investigate a multi-charter school system's (Harmony Public Schools [HPS]) after-school program in which doing a science fair project was expected for all 4th-12th grade students, and students were encouraged to participate STEM-related clubs (MATHCOUNTS, American Mathematics Competition [AMC], Science Olympiad, University Interscholastic League [UIL] and Science DEMO.). In particular, the first part of the study focused on how related matriculation of this specific school system was with the national average (n = 230). In the second part, the relationships between students' science fair and STEM club participation and their STEM major selection was investigated (n = 149). Findings showed that HPS outperformed the national average in terms of post-secondary admissions and STEM major selections. Multiple years of science fair project competition were positively related with students choosing a post-secondary STEM major (x[squared](4) = 5.32, p = 0. 255). There was also a statistically significant relationship between the number of STEM clubs students participated in and their choice of STEM major (x[squared](4) = 34.22, p less than 0.001). Findings are discussed in light of developing STEM-focused after-school clubs. (Contains 3 tables.)