Having friends influences students' academic, socio-emotional as well as health development. Especially, students with special educational needs (SEN) are at a high risk to belong to the subgroup of students without friends in class. Within a sequential explanatory design, two study phases have been implemented in the present study. In phase 1, a sociometric network analysis of 449 students from 25 inclusive primary school classes (4th grade, age = 9-11 years) were examined at t1. To assess students' friendships, sociometric nominations of the students' top five friends have been used. Results showed that the agreement of students' and teachers' rating varies strongly across classes (r = 0.18-0.71). In phase 2, interview data from a subsample (a sub-sample selected from phase 1) comprising teachers (n = 2) and students (n = 15) was qualitatively analysed. The evaluation of teachers' interviews identified three explanations for missing accuracy in the sociometric networks. From students' interviews, six subcategories describing friendship were found.