Background: The adoption of a shortened school week from the traditional 5 to 4 days is increasing nationwide. Budgetary and staffing needs are driving the change, yet research on the effects on students is lacking, especially regarding student health. Our study examined student health in 4 vs 5-day schools in Colorado, a state with one of the highest numbers of schools with a 4-day week. Methods: Data are from a population-based survey of high-school students. Student responses were weighted to school enrollment, and schools were classified by length of the school week. Indicators of health behaviors and outcomes were compared between the two groups, adjusted for student and school characteristics. Results: A 4-day school week was associated with several positive health outcomes (improved student engagement, decreased substance use behaviors, decreased non-school screen time, and increased physical activity) as well as several negative health outcomes (increased bullying, increased sexual activity, decreased sleep, and breakfast consumption). Conclusions: Our study found mixed health outcomes associated with attending schools with shortened weeks. The findings may be related to longer school days and a non-contact day, but further research is needed.