An international trend towards inclusive education is noticeable. However research shows that pupils with special educational needs (SEN) are at risk of becoming social rejected. This study addresses the reliability and discriminant validity of the Social Participation Questionnaire (SPQ), a teacher questionnaire to assess the social participation of pupils with SEN. In the first part the reliability and the discriminant validity was examined among 114 pupils with SEN and 114 typical peers in primary education. In the second part the reliability of the SPQ was studied among 108 pupils with SEN in secondary education. The questionnaire as a whole and each of the four subscales were found to be reliable and evidence was obtained for the discriminant validity. However, in secondary education, the interrater reliability of two subscales was not proven. Further research regarding the validity and the causes of the lack in interrater reliability in secondary education is recommended.