We examined the longitudinal relationship between sleep problems and behavioral problems at primary school in Italian children. We recruited a school-based sample of 227 children (age range 6–10 years) in schools located in Pisa (Italy). Parents completed the Sleep Disturbance Scale for Children (SDSC), and teachers completed the Strengths and Difficulties Questionnaire (SDQ) to evaluate children’s behavioral difficulties. We used a two-step robust regression approach to identify which aspects of sleep problems might affect children’s behavior at school. After removing socio-demographic effects, results indicated an association between an increase in sleep problems and the worsening of inattentive and hyperactive behavioral problems at school 1 year later. This association was particularly robust in children whose sleep problems had gotten worse over the year. We found no associations between child sleep problems and conduct problems in school settings. Schools may be a suitable arena in which to identify and prevent the development of severe externalizing behaviors through screening procedures and intervention for children’s sleep problems.