Vicarious contact has often been used for studying prejudice-reduction in school contexts due to its relatively accessible application through written or audiovisual portrayals of positive intergroup contact. However, these interventions may sometimes prove ineffective, thus restricting their ecological validity and independent use in education. To contribute to the understanding of factors that might facilitate or mitigate the efficacy of vicarious contact in reducing ethnic prejudice among adolescents, the present study tested for the moderating effect of anti-prejudice motivation and friends’ outgroup attitudes. Participants were Finnish secondary school students (N = 334; M = 13.38 years, SD = 0.53; 48% female; 19% ethnic minority) allocated into cluster-randomized intervention (N = 149) and control (N = 185) groups. Participants in the intervention group took part in 4 × 45-min teacher-led intervention sessions. A pretest-posttest design was employed to assess the outgroup attitudes three weeks before the intervention and the follow-up two weeks after. The results showed that adolescents’ intrinsic, but not extrinsic, anti-prejudice motivation and the pre-intervention attitudes of their reciprocal classroom friends positively predicted post-intervention attitudes towards people from different ethnic and cultural groups. However, only extrinsic motivation moderated the intervention effect as the results indicated the intervention to have a detrimental effect on outgroup attitudes among adolescents with less motivation to be non-prejudiced in order to gain social acceptance. This attitudinal backlash among adolescents less susceptible to the social influence of others implies that motivational aspects should not be overlooked when developing school-based intervention programs, especially when social norms are used as a mechanism of attitude change.