Transitioning Together is a multi-family intervention designed to support families of adolescents on the autism spectrum as they prepare for the transition to adulthood. Questions remain regarding its wide-scale adoption and implementation in real-world settings such as high schools. We examined student, teacher, and school-level facilitators and barriers to adopting and implementing Transitioning Together at 30 public high schools across three US states that were participating in the intervention arm of a larger randomized trial, which provided training and coaching for implementation of comprehensive evidence-based autism programming. Seventeen of the 30 schools adopted Transitioning Together. Community socioeconomic status and pre-existing quality of programming for family involvement and transition planning significantly predicted intervention adoption. Thirteen of the 17 schools that adopted Transitioning Together did so with a high level of fidelity (>90%). The areas of fidelity that schools struggled with most related to session structure, facilitating problem-solving and dialogue, and collecting feedback from families. Findings highlight struggles and successes with real-world adoption of the intervention in its current form. Future research is needed to further examine how to facilitate adoption across public high schools and/or other service systems, while maximizing effectiveness, as well as reach to historically underserved autism spectrum populations.