The Lone Star Stem (LSS) program was designed to increase high-quality STEM education opportunities and outcomes for high-need students in Texas. The focus of the program was on implementing rigorous coursework that helps students gain the skills, postsecondary credentials, and experience necessary to embark on well-paying careers in STEM fields. The LSS program also sought to increase participation in STEM dual enrollment coursework, improve educational outcomes for high-need students; build capacity in schools to offer STEM coursework, and provide high-quality resources on best practices in implementing STEM coursework. The evaluation of the LSS program used a mixed-methods approach; the research team examined (a) the impact of the program through a cluster randomized trial with randomization at the school level, (b) the extent to which schools implemented the program as intended, (c) scale-up initiatives that can be applied across Texas as well as to other states, and (d) the cost per student of developing and implementing the program. There were no significant main effects of the program on any outcomes, but moderator analyses indicate that the program was effective for some groups of students. Students in the treatment group saw significantly better outcomes than students in control schools in the areas of academic success, staying in school, and career readiness. Specifically, the likelihood of persistence toward graduation was significantly higher for Asian students and Indigenous students, rates of completing a career and technical education (CTE) concentration were higher among Asian and Black students, and Algebra II pass rates were higher for Indigenous students. All schools met the threshold for adequate implementation of the program, but further examination identified barriers to implementation, especially among rural communities, such as the inability to retain computer science teachers. Although the LSS program successfully scaled up efforts to disseminate best practices in implementing STEM coursework, considerations for future developers include nuanced fidelity measures for early correction of implementation challenges, particular attention to the unique challenges faced by rural communities in implementing programs like LSS, and implications for accessibility of grant funding to participating schools.