Four-day school weeks have proliferated across the United States in recent years, reaching over 650 public school districts in 24 states as of 2019, but little is known about their implementation and there is no consensus on their effects on students. This study uses district-level panel data from Oklahoma and a difference-in-differences research design to provide estimates of the causal effect of the 4-day school week on high school students' ACT scores, attendance, and disciplinary incidents during school. Results indicate that 4-day school weeks decrease per-pupil bullying incidents by approximately 39% and per-pupil fighting incidents by approximately 31%, but have no detectable effect on other incident types, ACT scores, or attendance.