summer melt
n. College-bound students who renege on their commitment to a particular school, especially during the time between high school graduation and the start of classes.

Example Citations:
Education researchers and academic counselors call it “summer melt,” the precarious time when some college-bound students fall through the cracks, at risk of abandoning their higher education plans entirely. Studies show that first-generation college students and those from low-income families are particularly vulnerable.
—Alan Scher Zagier, “Grads’ college plans often melt in summer,” The Commercial Appeal (Memphis, Tennessee), July 14, 2014

The college is currently admitting students from its wait list and final numbers won’t be known until fall, Staab said. There is also likely to be some “summer melt,” as students wait-listed at other colleges or universities get in elsewhere.
—Yvonne Zipp, “Kalamazoo College admits most diverse class in history, but falls short of enrollment goals,” Michigan Live, May 14, 2014

Earliest Citation:
At William and Mary, 1,140 of the 2,200 students offered admission have accepted. Another 100 students on the waiting list have since been offered admission and 75 of these have accepted, bringing the freshman class to 1,215. The college estimates an attrition of about 15 students, which officials refer to as “summer melt,” before fall, bringing the class right in on target.
—Deirdre Carmody. “Top Colleges Turn to Waiting Lists,” The New York Times, June 8, 1988

Notes:
A tip of the mortarboard to reader Lois Huffman for sending me this phrase. Also, word researcher extraordinaire Barry Popik has a 1984 citation.

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