In advanced economies, increasing population diversity often fuels hostile attitudes toward immigrants and political polarization. We study a short educational program for high-school students aimed at promoting cultural diversity and improving attitudes toward immigration through active learning. To identify the impact of the program, we designed a randomized controlled trial involving 4,500 students from 252 classes across 40 schools in northern Italy. The program led to more positive attitudes and behaviors toward immigrants, especially in more mixed classes. In terms of mechanisms, the intervention reduced students’ misperception and changed their perceived norms toward immigration, while it had no impact on implicit bias, empathy, or social contacts. Our findings suggest that anti-immigrant attitudes are primarily driven by sociotropic concerns rather than individual intergroup experience, and that educational programs combining critical thinking with cross-group discussion can correct them.