Purpose/Objectives: This study explores what school catering staff (nutrition program personnel) in the United Kingdom (UK) perceive to influence adolescents' food choices during the school day with observational research at school dining centers. Methods: Three middle schools in Wales, United Kingdom took part in the triangulated qualitative study. The research focused on pupils in Key Stage Three (aged 11 to 14 years old) enrolled at three schools. Data from interviews with the catering managers based at the schools (n=6), observations in school dining centers (n=3), and focus groups with catering staff (n=3) were thematically analyzed together. Results: Catering staff discussions and the observations revealed that the overall uptake of healthful foods was low in comparison to the number of pupils opting for unhealthful foods. Although healthful foods are available, pupils often avoided the most nutritionally balanced meal options. Catering staff perceived a multitude of factors as influencing adolescents' food choices at school: staff encouragement, peer pressure, parenting, education, health consciousness, vegan and vegetarianism, taste preferences, price consciousness, and convenience. Application to Child Nutrition Professionals: Exploring the perspective of catering staff or line level employees was a novel approach in better understanding the factors influencing adolescents' decision-making in the school food environment. These research findings are a beneficial starting point for further research or may potentially be used to influence how a healthful eating multifactorial policy could be implemented in middle schools. Of particular note, findings suggest a focus on ensuring that options are convenient and contain vegetables; avoiding overt labelling of vegetarian options; and introducing packed lunch regulations.