TURNER: The researchers who published their findings in the American Educational Research Journal poured over dozens of studies of learning time and arrived at a few other conclusions. While many districts have moved to a four-day school week, often because they believe it helps them hire and keep teachers, lead researcher Matthew Kraft at Brown University says the evidence here is really clear. MATTHEW KRAFT: Four-day school weeks are harmful for students learning and don't appear to be beneficial for teacher retention.