Mainstream teachers struggle with linguistic diversity, often leading to restricting multilingualism. Scientific research, however, recommends including pupils' home languages in school. Various qualitative studies have evaluated implementations in schools and indicated possibilities for improving teachers' attitudes towards multilingualism. This paper evaluates an experimental implementation targeting an increase in tolerance towards multilingualism. The implementation was facilitated by external school coaches and consisted of 3 experimental tools affecting the school. Data originated from 62 Flemish primary schools (of which half were experimental schools) that participated in 3 survey waves (2012 and 2014; 763 teachers completed both waves). We used multilevel regression. We concluded that the implementation leads to higher rates of tolerance. The fulfilment of the basic conditions needed for a successful change was important, and the linguistic diversity of the pupil population and the investment by external school coaches did not affect the tolerant practices of teachers towards multilingualism.