The aims of the current study were (i) to explore the extent to which pupils with Autistic Spectrum Disorders (ASD) were effectively included in lessons, compared with pupils with dyslexia (DYS) or no Special Educational Needs (CON) and (ii) to understand how the presence of a teaching assistant (TA) influences the inclusion/exclusion process. One hundred and twenty pupils (40 each in the ASD, DYS and CON groups) drawn from 12 mainstream secondary schools in the north-west of England were observed in a variety of lessons using structured observation schedules. Additionally, the classroom inclusion of 21 pupils with ASD was examined through qualitative observations. The analysis indicated that pupils with ASD were less effectively included in lessons than pupils in the other two groups. Specifically, they were less likely to work independently and be socially included, particularly when a TA was present. The findings are discussed in relation to the growing literature on inclusive education for pupils with ASD. (Contains 2 notes, 3 tables and 1 figure.)