In March 2020, many schools were closed to prevent the spread of COVID-19 in Japan, and it is predicted that many children, especially those with neurodevelopmental disorders (NDDs), will be affected emotionally and behaviorally. Here, we examined the impact of school closures due to COVID-19 on school-aged children with NDDs using the Child Behavior Checklist. Totally, data on 121 children diagnosed with autism spectrum disorder, attention-deficit hyperactivity disorder, and/or intellectual disorder were analyzed and it was found that externalizing and aggressive behavior increased in all NDDs, regardless of the type of diagnosis. A clear prospect is important for children with NDDs children to lead a stable life, and more generous supports for children with NDDs and their families are needed.