Due to the rising linguistic heterogeneity in schools, the inclusion of pupils with a first language other than the language of instruction is one of the major challenges of education systems all over the world. In this paper, attitudes of in-service teachers, pre-service teachers and parents towards the inclusion of pupils with a first language other than the language of instruction are examined. Additionally, as the paper focused on how the participants perceive the development of this pupils in different school settings (fully included, partly included, fully segregated). Data from 1501 participants were investigated. Descriptive results showed that pre-service teachers' attitudes towards the inclusive schooling of pupils with different language skills in composite classes were rather positive, while attitudes of in-service teachers and parents rather tend to be neutral. Regarding the results concerning the participants' attitudes towards the pupils' development in different school settings, all three sub-groups belief that pupils with German as first language would develop in a more positive way, compared to pupils without German as first language. Moreover, the migration background of pre-service teachers and parents had a positive influence on the participants' attitudes.