Children and adolescents with type 1 diabetes mellitus (T1DM) are at risk for a variety of problems at school. Well-controlled studies using data collected in schools, however, are limited. The purposes of this study are to determine whether selected school problems are associated with T1DM and to investigate an association between these problems and medical variables. Teachers rated 95 diabetic students (M = 11.8; SD = 3.0 yr old) and 95 of their siblings (M = 12.1; SD = 3.0 yr old) regarding academic skills, work completion, day-to-day variability, and classroom attention. Medical and school records also were accessed. The T1DM group had lower academic skills ratings overall (p < 0.02), especially in writing (p < 0.01), a trend toward poorer classroom attention (p < 0.08), and many more missed school days (p < 0.001). Diabetics on intensive therapy protocols had better academic ratings overall (p < 0.02), including in math (p < 0.03) and fewer missed school days (p < 0.03), but they unexpectedly were rated as having more classroom behaviors that jeopardize work completion (p < 0.05) than counterparts on conventional therapy. Among all diabetics, glycated hemoglobin (HbA(lc)) levels were moderately related to each academic skill rating (r = -0.34 to -0.37; p < 0.01) and strongly related to classroom attention (r = 0.53; p = 0.000). T1DM itself appears to be a relatively minor influence to several important aspects of school. Furthermore, although intensive therapy alone may well promote school success, meticulous glycemic control, however achieved, appears more important in mitigating prospective classroom attention and academic problems.