Aggression and externalisation problems at school hold negative consequences for both the perpetrators and the victims. This paper argues that the relationship between anxiety and aggression may make it possible to use certain anxiety prevention programmes to help prevent aggression. This hypothesis was tested on a sample of early adolescents using a quasi-experimental design with a control group. Four classes of early adolescents (N = 73) were randomly assigned to the control or intervention group. The intervention group was exposed at school to an anxiety prevention programme based on social and emotional learning called My FRIENDS. Aggression and externalisation problems were measured before the intervention, immediately after and at a six-month follow-up. The results show: (a) in the control group, aggression (dimensions: "aggression towards authority," "physical aggression") increased over the study period, whereas in the intervention group, it remained at the baseline level after the intervention and at the six-month follow-up; (b) externalisation problems (dimension: "hyperactivity") were reduced in the intervention group relative to the baseline after the intervention and at the six-month follow-up. The results support use of the My FRIENDS programme as a possible prevention programme for aggression and externalisation problems as well as anxiety. Study limitations and future research directions are considered.