The purpose of this study was to understand the impact, implementation and costs associated with a one-semester elective lab course in 9th grade, Accelerating Literacy for Adolescents (ALFA) Lab, which seeks to improve students' reading achievement, particularly for those from economically disadvantaged communities. This study used three cohorts of 9th grade students from four high schools in New York, Connecticut, California, and Alabama during the 2019-2022 school years to assess their reading achievement employing a regression discontinuity (RD) design. Students were identified for enrollment in the ALFA Lab course if their pre-ALFA Lab reading scores were below a cutoff. Reading achievement was measured using the comprehensive online literacy assessment STAR Reading, while reading motivation was defined using the Adaptive Reading Motivation Measure (ARMM) to obtain overall General Reading Motivation (GRM) and Reading Frequency (FRQ) scores. The four participating schools made considerable progress implementing the ALFA Lab program, as all observed ALFA Lab classes demonstrated evidence of implementing most ALFA Lab components included in the Checklist for ALFA Implementation (CFAI). Using a pooled analytic approach across cohorts and sites, ALFA students outperformed the BAU group in reading achievement (STAR assessment), reading motivation, and reading frequency (ARMM), though none of these were statistically significant. Mediation analyses revealed reading motivation and reading frequency explained 30% and 22%, respectively of estimated ALFA Lab impacts, though these effects were also non-significant. Moderation analyses found no statistically significant effects for STAR or reading motivation outcomes, but did find significant, positive effects for males, non-English Language Learner (ELL) students, non-special education (SPED) students, and Hispanic students. Average overall costs for the ALFA Lab program, as well as costs per student, are provided. Additional information presented includes an assessment of What Works Clearinghouse 5.0 Evidence for RD designs, mediation and moderation analytic approaches, implementation measurement details and participating school profiles.