Aims/hypothesis: We aimed to examine the association of type 1 diabetes with school wellbeing among Danish children. Methods: This is a population-based cohort study involving 436,439 Danish children, of which 1499 had a confirmed diagnosis of type 1 diabetes. The children were enrolled in grade levels 4 to 9 (middle school) in Danish public schools in the years 2014–2017. Questionnaire outcomes from the yearly National Wellbeing Survey related to self-efficacy, perceived competences, peer and teacher support, bullying and somatic symptoms were analysed. Ordered logistic regression was used to compare outcomes of children with and without type 1 diabetes, and to compare subgroups of children with type 1 diabetes by different levels of HbA_1c and diabetes duration. Primary outcomes were answers to seven pre-specified questionnaire items (scale, 1 to 5). Results: A total of 817,679 questionnaires were initiated, of which n = 2681 were from children with type 1 diabetes. Compared with the background population, children with type 1 diabetes expressed more peer support; adjusted OR 1.17 (95% CI 1.08, 1.27). Children with diabetes also reported more often having a headache; adjusted OR 1.09 (95% CI 1.00, 1.19). Overall, children with poor glycaemic control (HbA_1c >70 mmol/mol) had worse outcomes on the wellbeing measures compared with the background population. Even after adjusting for socioeconomic status, they still reported significantly worse perceived competences, less teacher support and more somatic symptoms (stomach ache and headache). Conclusions/interpretation: In Denmark, children with type 1 diabetes generally feel well supported in school but have more headaches than other children. Poor glycaemic control is associated with worse psychological school-related wellbeing. [Figure not available: see fulltext.].