Background: The Michigan Model for Health™ (MMH) is the official health curriculum for the State of Michigan and prevailing policy and practice has encouraged its adoption. Delivering evidence-based programs such as MMH with fidelity is essential to program effectiveness. Yet, most schools do meet state-designated fidelity requirements for implementation (delivering 80% or more of the curriculum). Methods: We collected online survey (N = 20) and in-person interview (N = 5) data investigating fidelity and factors related to implementation of the MMH curriculum from high school health teachers across high schools in one socioeconomically challenged Michigan county and key stakeholders. Results: We found that 68% of teachers did not meet state-identified standards of fidelity for curriculum delivery. Our results indicate that factors related to the context and implementation processes (eg, trainings) may be associated with fidelity. Teachers reported barriers to program delivery, including challenges with adapting the curriculum to suit their context, competing priorities, and meeting students' needs on key issues such as substance use and mental health issues. Conclusions: Multiple factors influence the fidelity of health curriculum delivery in schools serving low-income students. Investigating these factors guided by implementation science frameworks can inform use of implementation strategies to support and enhance curriculum delivery.