The study examined the Advancing Rural Computer Science (ARCS) program, a professional development initiative designed to improve elementary teachers' computer science (CS) content knowledge, pedagogical practices, and self-efficacy, with the goal of enhancing K-5 students' interest in and knowledge of CS. ARCS was funded through USED EIR U411C190032. The external evaluation employed a randomized controlled trial (RCT) design, with schools as the unit of random assignment. For two cohorts of schools, school teams were assigned to either the immediate treatment or delayed treatment (control) condition. The comparison condition (delayed treatment/control) was business-as-usual instruction, where teachers received no additional CS-focused professional development (PD) beyond standard district offerings. The program was implemented without substantive adaptations during the study. The study was conducted in elementary schools located across Virginia, with a focus on serving students in rural divisions and who are traditionally underrepresented in STEM and CS education, including Black, Hispanic, and mixed-race students, students from economically disadvantaged backgrounds, and those in geographically isolated communities. The sample included K-5 teachers and their students. Teachers in schools randomized in the treatment condition completed a 5-day summer academy and academic follow up in year 1 and microcredentialing and academic year follow up in year 2 of the intervention and engaged in professional learning communities across the two years of the intervention. Teachers in schools randomized into the control condition were eligible to participate in the two-year intervention the year following their control year. For both cohorts of schools, key Year 1 outcomes measured included teachers' CS content and pedagogical knowledge, CS self-efficacy, and instructional implementation frequency, and students' CS content knowledge and interest in CS. Results indicate that for two cohorts of schools, one year of ARCS participation improved student CS content knowledge; controlling for all model covariates. The treatment effect indicated that treatment school student content knowledge means were 0.872 points greater than control group schools ([gamma subscript OO] = 10.10); [gamma subscript 01] = 0.872, p = 0.0005, g = 0.186. However, there was no statistically significant improvement in student CS interest; controlling for all model covariates, the treatment effect indicated that treatment school means were 0.508 points greater than control group schools ([gamma subscript OO] = 8.24); [gamma subscript 01] = 0.508, p = 0.057, g = 0.036. Results indicated that for two cohorts of schools, one year of ARCS participation increased teachers' CS content knowledge and self-efficacy for teaching CS. Controlling for all model covariates, the treatment effect indicated that treatment school teacher content knowledge means were 1.50 points greater than control group schools ([gamma subscript OO] = 9.04); [gamma subscript 01] = 1.50, p = 0.001, g = 0.513. Controlling for all model covariates, the treatment effect indicated that treatment school teacher self-efficacy means were 8.11 points greater than control group schools ([gamma subscript OO] = 9.25); [gamma subscript 01] = 8.11, p < 0.001, g = 0.221.