The aim of this paper is to report on the lunchtime food purchasing practices of secondary school students and some of the factors related to this purchasing, including the influence of socio-economic status (SES) and the food environment within and around schools. A mixed-methods study incorporating an online purchasing recall questionnaire and multiple qualitative methods was undertaken at seven UK secondary schools. The analysis shows that SES was intricately woven with lunchtime food practices. Three-quarters of participants regularly purchased food outside of school; those at low SES schools were more likely to report regularly leaving school to buy food. Young people's perception of food sold in schools in areas of low SES was often negative and they left school to find "better" food and value for money. Taste, ingredients and advertisements were factors that mattered to young people at schools with low or mixed SES; health as a driver was only mentioned by pupils at a high SES school. For public health initiatives to be effective, it is critical to consider food purchasing practices as complex socio-economically driven phenomena and this study offers important insights along with suggestions for designing interventions that consider SES. Availability of food outlets may be less important than meeting young people's desires for tasty food and positive relationships with peers, caterers and retailers, all shaped by SES. Innovative ways to engage young people, taking account of SES, are required.