The purpose of this study was to evaluate Peer Group Connection-Middle School, a cross-age peer mentor program designed to enhance student retention and achievement during the transition from elementary to middle school. We present findings from an individual-level randomized controlled trial that included 1,902 sixth-grade students across three cohorts (2018-19, 2019-20, and 2021-22) from three Eastern states. We estimated program impact within the intent-to-treat (ITT) framework for two confirmatory outcomes, number of days in attendance and number of classes passed in sixth grade. Impact estimates were calculated using a regression equation that models the outcome of interest as a function of treatment status, and a series of covariates, including the baseline measure of the outcome variable. Findings from a summative implementation evaluation, which aimed to explore the extent to which the PGC-MS program was implemented as intended are also presented in this report. Benchmark findings indicate that offering the PGC-MS program to sixth-grade students did not have a detectable impact on the confirmatory outcomes of attendance or the number of passed classes during the first year of middle school. Implementation data suggest there was considerable variation in the implementation of PGC-MS across study schools and cohorts, which resulted in low dosage and a diluted treatment-control contrast. The second cohort was halted by the onset of the COVID-19 pandemic in March 2020, and study schools from the third cohort reported continued challenges with implementation during the 2021-22 school year. PGC-MS was a new, innovative program model at the time this evaluation took place. Thus, future research should examine the potential for a well- and consistently implemented program to have a positive impact on outcomes as students make the transition from elementary to middle school. [This report was submitted by the Policy & Research Group.]