BACKGROUND: Sleep problems and daytime sleepiness are common in children with attention-deficit hyperactivity disorder (ADHD) and are associated with poor parent-reported functional outcomes. However, the potential impact of sleep problems or daytime sleepiness on the school functioning of children with ADHD remains unknown. We aimed to determine if sleep problems and daytime sleepiness were associated with the social, emotional, and behavioral school-based functioning of children with ADHD and comorbid sleep problems. METHODS: Children aged 5-13 years with ADHD and a moderate-severe sleep problem (confirmed using American Academy of Sleep Medicine diagnostic criteria) were recruited from 43 pediatric practices across Victoria and Queensland, Australia (N = 257). Parent-rated sleep problems were assessed using the Children’s Sleep Habits Questionnaire (CSHQ) and teacher-rated daytime sleepiness using the Teacher’s Daytime Sleepiness Questionnaire. Teacher-rated social, emotional, and behavioral school functioning was assessed using three scales (peer problems, emotional problems, and conduct problems) from the Strength and Difficulties Questionnaire. Data was analyzed using Pearson correlations and linear regression models. RESULTS: Teacher-rated daytime sleepiness was associated with higher levels of emotional (beta = 0.39; 95% CI = 0.25-0.52) and behavioral problems (beta = 0.47; CI = 0.36-0.58) in adjusted models. While total sleep duration and parent-rated sleep problems were not associated with daytime sleepiness or school functioning, the CSHQ subscale night wakings was correlated with teacher-rated daytime sleepiness (r = 0.21; p < 0.01). CONCLUSIONS: Daytime sleepiness (possibly as an indicator of sleep quality) may be a better predictor of school functioning in children with ADHD who have concomitant sleep problems than total sleep duration or parent-rated sleep problems.