By the end of middle school, students may lose a third to half of what they learn during the year to the so-called “summer slide.” That’s because what students forget over the summer becomes more damaging over time as students make less progress during the school year, according to an ongoing study by NWEA, the Northwest Evaluation Association. “As kids grow less and less during the school years, they are still seeing the same summer drop—so they are losing proportionately more,” said Megan Kuhfeld, a research scientist at NWEA and the author of the study.