ASD symptomology and behavioral problems pose challenges for children with ASD in school. Disagreement between parents and teachers in ratings of children’s behavior problems may provide clinically relevant information. We examined parent–teacher disagreement on ratings of behavior problems among children with ASD during the fall and spring of the school year. When child, teacher, and class characteristics were considered simultaneously, only ASD symptom severity predicted informant disagreement on internalizing and externalizing behavior problems. We also examined associations between informant disagreement and parent school involvement. Cross-lagged panel analyses revealed that higher informant disagreement on children’s behavior problems in the fall predicted lower parent school involvement in the spring, suggesting that greater informant agreement may foster parental school involvement over time.