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. Lay abstract Transitioning Together is an intervention that supports families of adolescents on the autism spectrum as they prepare for the transition to adulthood. While it has been delivered successfully and shown to result in positive outcomes for families in the university setting, questions remain about whether and how well it can be widely provided to families in real-world settings such as high schools. In this study, we analyzed predictors, facilitators, and barriers to providing Transitioning Together to families at 30 high schools across three US states, all of which received training from a team of researchers to deliver this intervention. Our findings highlight struggles and successes with real-world use of the intervention. Seventeen of the 30 schools were successful in providing Transitioning Together to families. Schools who had higher community socioeconomic status, higher quality family involvement, and higher quality transition planning programming before changing anything for this study were much more likely to provide this new intervention to families. Schools who used the intervention were mostly able to deliver it as designed and received positive feedback from families who participated. Common parts of the intervention that schools struggled with most included following the structure of the sessions, including group problem-solving and dialogue in the sessions, and collecting feedback from families. Future research is needed to learn how to make it even easier for public high schools and other service systems to provide this intervention to families, in a way that also maximizes its effectiveness and accessibility for historically underserved autism spectrum populations.