Academic achievement under a four-day school week is compared to the traditional five-day school week. Test scores from the CAT [California Achievement Test], ITBS [Iowa Test of Basic Skills], TASK [Stanford Test of Academic Skills], SAT [Stanford Achievement Test], TAP [Tests of Academic Proficiency], and MAT [Metropolitan Achievement Test] were analyzed for a fifteen year period: the five years before the four-day school week, five years on the four-day school week, and then the five-years immediately off the four-day school week. Adequate Yearly Progress (AYP) was measured in grades eight, eleven, and twelve in reading, mathematics, and language subject areas. The data support the interpretation that the majority of students made AYP across the fifteen year period, with no detrimental effects to standardized test score results.