Ah, those last golden weeks of senior year in high school—remember them? College applications blitzed the fall. Worry clouded the spring. But by mid-April, acceptance letters had come through, and it was time for a high school tradition right up there with Homecoming and the Prom: happy, decadent Senior Slump. You blew off homework. You pulled a prank or two. So what? You were in college, after all, and that was that.

But, say college counselors, better beware passing on this tradition if you’ve got a kid in high school today. Why? Things have changed: admissions are tougher than ever, especially for students applying to the top 150 colleges. “We estimate that over three quarters of the students who apply for admission to Yale are qualified to do the work here,” says the Office of Admissions. However, the actual acceptance rate is a mere, and terrifying, 9%.

And what does this mean for today’s high school seniors? If your child is lucky enough to get one of those golden letters in the mail, it's not the end of the story for college admissions departments. “Once a student graduates from high school,” says Lisa Sohmer, Director of College Counseling at the Garden School in New York, “a final transcript is sent to the college where the student will enroll. Make no mistake—these records are carefully reviewed.”

And what then? “Colleges want to be sure,” she adds, “that the student who was admitted in April is the same student who will arrive on campus in September.” If a student’s transcript shows a big dip—either in the level of classes taken or in actual grades—it’s very possible, as she puts it, that “acceptances can be revoked or modified.” And for every spot that opens up at competitive colleges, lots of highly qualified kids are lined up.

Of course, your high school senior will probably crave a chance to relax a bit at the end of the year. “If seniors ask about reducing the strength of their second semester schedule,” says Carl Peterson, counselor at Forest Hills Eastern High School in Ada, Michigan, “I’ll always advise them to contact any colleges to which they’ve applied, since the colleges decision is based in part on the senior course schedule.” College advisors can help evaluate programs and see what will be most important for the next step.

The bottom line? Seniors should be listening to their advisors, not to Ferris Bueller. Sohmer says, “The message to students is, ‘senior slump’ is a thing of the past.”