IMPORTANCE Type 1 diabetes has been associated with cardiovascular disease and late complications such as retinopathy and nephropathy. However, it is unclear whether there is an association between type 1 diabetes and school performance in children. OBJECTIVE To compare standardized reading and mathematics test scores of schoolchildren with type 1 diabetes vs those without diabetes. DESIGN, SETTING, AND PARTICIPANTS Population-based retrospective cohort study from January 1, 2011, to December 31, 2015 (end date of follow-up), including Danish public schoolchildren attending grades 2, 3, 4, 6, and 8. Test scores were obtained in math (n = 524 764) and reading (n = 1 037 006). Linear regression models compared outcomes with and without adjustment for socioeconomic characteristics. EXPOSURES Type 1 diabetes. MAIN OUTCOMES AND MEASURES Primary outcomes were pooled test scores in math and reading (range, 1-100). RESULTS Among 631 620 included public schoolchildren, the mean (SD) age was 10.31 (SD, 2.42) years, and 51% were male; 2031 had a confirmed diagnosis of type 1 diabetes. Overall, the mean combined score in math and reading was 56.11 (SD, 24.93). There were no significant differences in test scores found between children with type 1 diabetes (mean, 56.56) and children without diabetes (mean, 56.11; difference, 0.45 [95% CI, −0.31 to 1.22]). The estimated difference in test scores between children with and without type 1 diabetes from a linear regression model with adjustment for grade, test topic, and year was 0.24 (95% CI, −0.90 to 1.39) and 0.45 (95% CI, −0.58 to 1.49) with additional adjustment for socioeconomic status. CONCLUSIONS AND RELEVANCE Among Danish public schoolchildren, there was no significant difference in standardized reading and mathematics test scores of children with type 1 diabetes compared with test scores of children without diabetes.