British children have the highest levels of ultra-processed food (UPF) consumption in Europe. Schools are posited as a positive setting for impacting dietary intake but the level of UPFs consumed at schools is currently unknown. This study determined the UPF content of school food in the UK. We conducted a pooled cross-sectional analysis of primary (4-11 years, n=1,895) and secondary schoolchildren (11-18 years, n=1,408) from the UK’s National Diet and Nutrition Survey (2008-2017). Multivariable quantile regression models determined the association between meal-type (school meal or packed lunch) and lunchtime UPF intake (NOVA food classification system). We showed that on average UPF intake was high in both primary (72.6% total lunch Kcal) and secondary schoolchildren (77.8 % total lunch Kcal). Higher UPF intakes were observed in packed lunch consumers, secondary schoolchildren, and those in lower income households. This study highlights the need for a renewed focus on school food. Better guidance and policies which consider levels of industrial processing in food served in schools is needed to ensure the dual benefit of encouraging school meal uptake and equitably improving children’s diet.