This study explores the impact of four-day school weeks on early elementary achievement. Using covariate adjusted regression analyses and data on all students who entered kindergarten in Oregon, USA between 2014 and 2016, we examine differences in 3rd grade math and English Language Arts test scores (i.e., achievement) for students enrolled in a four-day school week versus a five-day school week at kindergarten entry. On average, we find minimal differences between 3rd grade test scores of four-day and five-day students, but there are notable differential effects across the spectrum of these students' kindergarten readiness scores and educational program participation. We find that above median performers on kindergarten assessments, White students, general education students, and gifted students - student groups that make up more than half our sample - are the most negatively impacted by the four-day school week during the early elementary period. We generally find no statistically significant evidence of detrimental four-day school week achievement impacts for students who were below median performers on kindergarten assessments, minority students, economically disadvantaged students, special education participants, and English as a second language students.
Keywords: achievement; four-day school weeks; kindergarten assessment.