The need for well-implemented evidence-based interventions (EBIs) for the prevention of behavioral issues among children and adolescents is substantial. In rural areas, the need often matches or surpasses that of urban areas. Schools have a wide reach for prevention-focused EBIs. However, implementation in rural schools is often hindered by limited resources and capacity. Rural School Support Strategies (RS3) are a bundle of implementation supports that address implementation challenges in rural settings. They include providing additional leadership and coaching training, individualized technical assistance (mostly virtual), and monthly meetings of a virtual learning collaborative. A cluster-randomized Hybrid Type 3 implementation-effectiveness trial tested RS3 for implementing school-wide positive behavioral interventions and supports (PBIS), a universal prevention approach to improving student behavior, academic outcomes, and school climate. Forty rural schools received a multi-day training on PBIS each summer for 3 years. Half were randomized to also receive RS3 support. Linear and logistic regression models examined the effect of treatment condition and dosage of support on implementation fidelity for Tier 1 (universal) PBIS. Condition and dosage (number of hours) of support increased the odds of schools achieving the 70% threshold for adequate implementation fidelity. In the first year, the higher dosage of technical assistance events increased the likelihood of schools reaching fidelity, whereas later in the trial, the higher dosage of attendance at the virtual learning collaborative sessions yielded significant improvements in fidelity. Implications for accelerating the implementation of universal prevention initiatives in schools--particularly in rural settings--are discussed. This study was prospectively registered on ClinicalTrials.gov (NCT03736395), on November 9, 2018.