Background: School meals were served free of charge to all public school students in the United States during the COVID-19 pandemic, but some students still did not participate. Methods: In this mixed-methods study, surveys and interviews were conducted with food service directors (FSDs) from California (n = 556 surveys; n = 29 interviews) and Maine (n = 43 surveys; n = 20 interviews) during spring 2022. Survey data was analyzed using multivariable logistic regression models, and interview data was analyzed using the immersion/crystallization approach. Results: Students' preference to eat meals from home or elsewhere (81.5%) and negative perceptions of the school food's taste (67%) were the most common barriers reported. Schools' prior community eligibility provision (CEP) participation and smaller student enrollment were associated with fewer reported barriers. Inadequate time to eat lunch and stigma were also reported as barriers to participation. Implications for School Health Policy, Practice, and Equity: Investments are needed to help schools partner with students to optimize school meal experiences and to improve food taste. Policies that provide school meals free of charge for all students can also help reduce negative social barriers such as stigma. Conclusions: Despite the provision of USM, barriers to participation remain. Policies to enhance meal quality and program implementation are needed.