Background: Translating research findings into practice should be one of the objectives of developmental psychology. Recently, research demonstrated the existence of individual differences in theory of mind (ToM) during middle childhood that are crucial for children's academic and social adjustment. Aims: This study aims to transfer the results of recent experimental studies on ToM interventions into primary-school teachers' practices. It examines whether a ToM training programme, based on conversations about mental states, can be effective under real-world school conditions and if it can be translated in such a way that it becomes suitable for primary-school teachers. Sample: Seventy-two 8- to 9-year-old children took part in the study. A total of four classes were recruited and randomly assigned to the experimental (34 children, 18 boys) or to the control condition (38 children, 18 boys). Methods: The ToM group and the control group were matched at pre-test for age, ToM, socio-economic background, verbal ability, working memory, and planning. Teachers were trained in four teacher-training sessions; the classroom-training programme was delivered by teachers in four sessions (each 50 min long). Children were assessed before the intervention, after the end of the programme, and 2 months later. Results: The ToM group improved ToM skills significantly more than the control group both in the short and in the long term. Conclusions: Teachers can successfully promote their pupils' ToM development during their regular teaching hours. Results are discussed in the light of the importance of ToM promotion for children's school adjustment.