In a non‐randomized controlled study, we investigated the efficacy of a school‐based mindfulness curriculum delivered by schoolteachers to older secondary school students (16–18 years). We measured changes in emotion processing indexed by P3b event‐related potential (ERP) modulations in an affective oddball task using static human faces. ERPs were recorded to happy and sad face oddballs presented in a stimulus stream of frequent faces with neutral expression, before and after 8 weeks of mindfulness training. Whilst the mean amplitude of the P3b, an ERP component typically elicited by infrequent oddballs, decreased between testing sessions in the control group, it remained unchanged in the training group. Significant increases in self‐reported well‐being and fewer doctor visits for mental health support were also reported in the training group as compared to controls. The observed habituation to emotional stimuli in controls thus contrasted with maintained sensitivity in mindfulness‐trained students. These results suggest that in‐school mindfulness training for adolescents has scope for increasing awareness of socially relevant emotional stimuli, irrespective of valence, and thus may decrease vulnerability to depression. This non‐randomised controlled study examined changes in P3b brain potentials of 16‐18 year olds from before to after an 8‐week mindfulness programme. Mindfulness training resulted in maintained P3b amplitudes to sad, happy and neutral faces in the training group only, indicating sustained sensitivity to socially relevant stimuli.