The aim of this study was to explore whether the classroom implementation of the International Child Development Programme (ICDP) for secondary school students (grade 9) was linked to better school achievement. The goal of the ICDP is to increase school achievement by promoting positive teacher-student relationships. The study, performed in Sweden, applied a pre--post design (four years) with matched intervention and control schools (N = 148). The post-intervention assessments showed that there were significant differences in school achievement in Grade 9 between the intervention school and the control school. Specifically, a greater proportion of students at the intervention school demonstrated improvement in school subjects and achieved the competency requirements to enter an upper secondary school programme. Based on the results, the ICDP can be considered an important intervention to promote student learning by promoting positive teacher-student relationships.