Negative peer attitudes are a significant barrier to social participation of students with disabilities in inclusive classrooms. Although many intervention studies have demonstrated the effectiveness of interaction or disability awareness programmes in promoting positive peer attitudes, reliance on students' self-reported attitudes is prone to social desirability bias and is unable to capture implicit prejudice. The present study examined changes in students' explicit and implicit attitudes toward peers with disabilities after a curriculum-based intervention programme ("Prinzip Vielfalt"), which aims to promote an open-minded attitude toward human diversity. Teachers in 12 primary school classes in the experimental group (n = 195 students) used the programme for eight weeks, whereas a control group of 12 classes (n = 191 students) used no intervention. Students' pre- and post-intervention attitudes were assessed using an adapted version of the Chedoke--McMaster Attitudes Towards Children with Handicaps Scale and a disability Implicit Association Test for children. Post-intervention analyses revealed a significant relative improvement in explicit attitudes toward peers with disabilities in the experimental group compared to the control group (b = 0.20, 95% CI = [0.03; 0.37]) but no change in implicit attitudes (b = -0.03, 95% CI = [-0.10; 0.03]). Thus, while the intervention positively affected self-reported attitudes, implicit negative associations were unchanged. Using explicit and implicit measures of attitudes and attitude changes among children can provide valuable insights into the mechanisms and effectiveness of different intervention strategies.