Positive social relationships are essential for supporting adolescents in multicultural schools. This preregistered study quantitatively examined the effectiveness of the Peer Integration and Enhancement Resource (PIER), a school-based psychosocial intervention, in increasing adolescents' social capital; it also qualitatively analyzed teachers' experiences of the PIER. Adolescents with migrant backgrounds and those with native backgrounds were randomly allocated to the intervention and control groups in England (N = 108) and Finland (N = 370). Indicators of social capital, e.g. peer relationship quality, sense of belonging, and prosocial behavior, were assessed at baseline, post-intervention, and at follow-up. The quantitative results demonstrated that the PIER did not significantly improve adolescents' social capital. The qualitative analyses of the teachers' focus group discussions identified four main themes: emotional support through experiential processes, security created through exercises, identity building through exploration, and connection through group dynamics. Tailoring the intervention to meet adolescents' needs would improve its effectiveness.