The attitude of teachers towards inclusive education is a key issue for the success of inclusive education. Many studies have been designed to assess teachers' attitudes, but none have looked at the bias caused by teachers' non-response to questionnaires on their attitudes. Non-response biases are difficult to identify because it is impossible to access the responses of individuals who do not wish to reply. It is therefore necessary to use other data. This article examines the non-response biases of teachers in questionnaires about their attitude towards inclusive education by cross-checking the responses of parents of children with a disability (N = 382) regarding the quality of their child's inclusion and the responses or non-responses of teachers to questionnaires about their attitude (N = 48 responses). This study shows that the non-responses of teachers are not random and are associated with poorer well-being and social inclusion at school, a lower sense of belonging and higher levels of bullying. This significant non-response bias of teachers leads us to question the existence of a potential positivity bias in previous studies.