Many children in bilingual regions follow lessons in a language at school (school-language) that they hardly ever speak at home or in other informal settings. What are the neural effects of this phenomenon? This functional magnetic resonance imaging (fMRI) study investigates the effects of using school-language on brain activity during a high cognitive task in two groups of French/Dutch bilingual children. Participants were asked to assess the correctness of 3-operand (e.g., 5 − 2 = 3) and 4-operand (e.g., 3 − 2 = 1 + 4) equations, respectively. Instructions about the task were given either in the children's school-language or in another language that they only used at home. It was found that although both groups recruited a visuomotor occipitofrontal network in the left hemisphere, the children who performed the task in their school-language showed less activation than the children who did not, indicating the importance of language of instruction for bilingual children's arithmetic processing.