Since 1978, support for inclusive education in English mainstream schools has grown tremendously. Yet the inclusion of children with Special Education Needs (SEN), and especially Emotional and Behavioural Difficulties (EBD), has only had modest success. This is mainly because the belief that such children detract from the academic performance of a school is systemic. The literature does not say that this is indeed the case; it is equivocal. This study examines (1) the effect of an EBD school environment on individual children, (2) the typical attributes of a school environment in which EBD flourishes and (3) the impact of the presence of children with SEN on Local Educational Authority outcomes. The methodology is generally quantitative, relying mainly on Multivariate Data Analysis techniques and Data Envelopment Analysis. The results indicate that the majority of children with EBD are likely to be affected by the environment in an EBD school. Both EBD and SEN can impact on mainstream performance but while SEN may directly influence performance like a socio-economic constraint, the impact of EBD is likely to be indirectly on the school environment. This difference does not, however, explain why more children with EBD are segregated. Rather the indications have support previous research that suggests children with perceived EBD are segregated for reasons other than the difficulties per se.