Some high school seniors may have scoffed at warnings about partying instead of studying this spring. But nagging counselors and parents turn out to have been right: A senior year slump can have painful repercussions.

In June and July, elite universities across the United States increasingly are revoking admission offers to students whose grades originally were good enough to gain acceptance, but whose final exams and transcripts took a dive into Ds or worse. It's a little-known practice, but it can dump as much as 2 percent of an incoming class.

For example, the University of California at Los Angeles has begun to send out letters informing some students that their "academic record no longer meets the standards for admission." So, the coveted acceptances to the freshman class, celebrated just months ago, are withdrawn. Gone. Revoked. Frittered away.

"It can be quite traumatic," Susan Wilbur, the University of California system's director of undergraduate admissions, said of the revocations' effect on students and their parents. The early-summer timing is especially hard, she said, because the student usually has turned down other admissions offers by then and has few options left at four-year colleges.

But with so many strong applicants previously rejected at competitive campuses, "it is absolutely incumbent upon us to uphold the integrity of the process and maintain the high standards," Ms. Wilbur explained.

Universities say they are open to appeals about special circumstances, such as an illness or a divorce that affected grades. They may forgive an otherwise-stellar student who stumbles in one ambitious course.

And some, especially private universities not bound by state entrance formulas, will allow students to repeat courses in summer school, delay admission for a year or admit them on a probationary basis.

Still, the increasing competition at elite schools is making some institutions less lenient of senioritis, and more willing to eject a student who already sent in an enrollment deposit, said Barmak Nassirian, associate executive director of the American Association of Collegiate Registrars and Admissions Officers. "Schools are becoming more stern about that than they were in the past," he said. "If it is a case of [a student's] deciding that 12th grade was a time for merriment, it is hard to cut those kids some slack in these competitive times."

The numbers of such reversals are not large, but many Southern California high schools report at least a couple of students worried sick about a final D or F in a required course. UCLA already has revoked about 25 freshman admissions in recent weeks and expects to withdraw about 90 by midsummer, as final high school transcripts arrive for its expected 4,600 freshmen, according to Vu T. Tran, UCLA's director of undergraduate admissions.

Because application deadlines were in November, grades in senior classes are not used in initial admissions decisions, he said. Still, students are expected to maintain a B average in their senior year, and not to score below a C in any of their major courses, especially the ones required for entrance, he said.

UCLA reviews each case individually and may show leeway if the trouble was with just one course, and there were mitigating factors, Mr. Tran said. "We are not cold-hearted," he stressed. But he, too, spoke about the need "to be equitable and fair, not only for the students we admitted, but also for the ones we denied."

High school counselors said they often warn students that college acceptances are conditional, based on keeping up their grades. But the message does not always get through. Embarrassed about their predicament, most revoked students are reluctant to discuss it.

Private universities say they can be more flexible than state schools.

At the University of Pennsylvania, a private Ivy League institution, Dean of Admissions Lee Stetson said his office looks at final transcripts and is more likely to send out a warning about declining high school grades than a revocation. Such students usually are told that they will be closely watched their freshman year.

Some senioritis is understandable, Mr. Stetson said. "If you've already gotten into the school of your choice, it's human nature to let up a little bit. It's only natural. On the hand, it's the degree, the pattern, that is the issue."