This study examined teacher behaviours, students' academic engagement, and classroom ecology in mainstream classrooms including children with intellectual disabilities, and examined the differences in academic engagement for students with and without intellectual disabilities. A mixed-method research methodology was employed. The study demonstrated that children with intellectual disabilities exhibited high level of engagement in the following situations: the classroom layout used grouping, the classroom organisation combined groups and independent learning, the learning tasks included reading aloud, copying, watching videos, and doing homework, and teachers paid attention to all students or to children with intellectual disabilities individually and walked around the classroom during lesson time. English was the discipline with most active engagement among children with intellectual disabilities, followed by music, art, science, Chinese, sports, social studies, and mathematics. Finally, academic engagement of children with and without intellectual disabilities was mostly passive. We discuss the factors related to academic engagement of children with intellectual disabilities and provide suggestions for improving their self-management skills and classroom organisation.