The aim of this field experiment was to develop and assess an intervention promoting positive intergroup relations in culturally diverse schools. The intervention was based on extended contact and social learning and utilized behavioural journalism as its method. Intervention effects were assessed on out-group attitudes, perceived importance of future contact, perceived peer norms and intergroup anxiety among ethnic majority (N = 583) and minority (N = 214) youth in grades 7-9 in Finnish secondary schools (total N experimental = 388; total N control = 409). As a result of the intervention, both groups showed a tendency to perceive future intergroup contact as more important, and this effect was most notable for younger participants and girls. Prototypicality of in-group and out-group peer models contributed positively to intervention effects. However, unexpectedly, the intervention also increased experiences of intergroup anxiety among the oldest participants. The results are discussed taking into account the developmental stage of the youth studied. Besides critically assessing the effectiveness of the intervention, recommendations for improving theory-driven prejudice-reduction and for the development of future interventions in culturally diverse contexts are given.