summer slide

summer slide
 n.— «Yes, many children fall almost three months behind in math and reading skills over the summer. This phenomenon is so well known that educators even have a special name for it. They call it the “the summer slide.” Because of the summer slide teachers often invest the first two months of every school year focusing on lesson plans that help students regain skills they lost over the summer.» —“Avoiding the Summer Brain Drain” by Thomas Haller, Chick Moorman ModernMom June 19, 2007. (source: Double-Tongued Dictionary)