Effects of a mindfulness-based program, Compassion and Attention in the Schools (Compas), were studied in 358 pupils in grades 5, 7, and 8 in Sweden. An experimental group undertook Compas practices in class three times a week during an eight-week period. A control group undertook content area academic lessons . Pre-/post-intervention analyses showed a significant improvement in the experimental group, but not in the control group, in pupils' capacity for effortful control, feelings of well-being at school and perceived peer relations. The positive effect of training increased with the number of times the participants took part in the training for all but one of the measures (general stress). Compas seems to be a useful tool for enhancing pupils' effortful control, well-being at school and peer relations.