Emotion dysregulation is a common social functioning deficit in children with attention-deficit/hyperactivity disorder (ADHD). Little is known about how teachers rate irritability in school-aged children with ADHD. We examined irritability and disruptive behaviour in clinic-referred school-aged children (n = 145) with ADHD. Children in this cross-sectional study were 6-12 years old (M[subscript age] = 9.2 years, 16.6% female). We obtained ratings from children themselves as well as their mothers, fathers, and teachers at the baseline of a family intervention program. Descriptive statistics and multilevel regression models were employed. Children displayed increased irritability, such as frequent temper outbursts, when compared with population sample peers. Children's proneness to anger was strongly associated with perceived peer difficulties particularly in teacher ratings. Findings highlight the significance of home- and school-based support when considering social functioning in children with neurodevelopmental conditions.