Wiktionary
n. The phenomenon in which incoming college students who have committed to attend a college/university decide during the summer to change their minds and attend another institution. This sometimes happens when students are admitted over the summer to a school that place the student on a waitlist in the spring.
Wikipedia
Summer melt is the phenomenon of prospective college students' motivation to attend college "melting" away during the summer between the end of high school and beginning of college. This phenomenon is especially prevalent in low-income minority communities, where students who qualify for college and in some cases even register for classes ultimately end up not attending college because they lack resources, support, guidance, and encouragement. Causes include the costs associated with college and a lack of support. Support programs, particularly by colleges, have been found to reduce summer melt.
Usage examples of "summer melt".
Slumps occurred and cave-ins as the surface warmed, and where the summer melt could find no outlet, bogs and swamps and thaw-lakes appeared.
The permanently frozen ground beneath the surface layer, softened now by spring and summer melt, allowed no drainage to percolate through.
The Avenue here was on two levels, a fairly deep center filled with crystal-blue water that allowed the summer melt to drain off, creating the Quilst swamp far to the south.