Every school day, more than 95% of the nation's five- to 17-year-olds attend one of almost 110,000 elementary and secondary schools where they receive instruction in how to avoid major health-threatening behaviors. School is also the place where children and adolescents can be given preventive health care services, including administration of vaccines and monitoring of immunization levels. This article addresses how expanding school health services could improve immunization levels of school-aged youth.