Slackers, beware! That fat envelope is conditional. If your grades drop, some colleges won't hesitate to take it back.

• Author: Pappano, Laura
• Position: COLLEGE

You may have gotten a thick envelope with a congratulatory letter from the college admissions office. You may have told everyone where you're going. You're in, but remember: You're not done. After being accepted at Franklin & Marshall College, in Lancaster, Pa., Isa Valera spent her last semester of high school doing everything except studying. When she wasn't at her two jobs, she focused on the prom and graduation from Frederick Douglass Academy in the Bronx, N.Y. Her grades fell from 80s and 90s to "barely passing."

Just weeks before classes began, the college got in touch: Admissions was rethinking her acceptance.

Valera was ultimately allowed to enroll, but only after she had written a contrite letter and an additional essay, and agreed to meet monthly with the dean of admissions.

"Senioritis" has infected the college-bound since, oh, forever. But with record numbers of applicants and long waiting lists, colleges are in a better position to insist that students stay focused. More schools now check midyear and final transcripts, mailing warnings or calling students with fallen averages. With some colleges rescinding admission offers, the message that a college acceptance is conditional is finaLly getting through.

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The University of Colorado at Boulder rescinded admission for 45 students in 2006. The...