The students received a congratulatory note and a bright red UGA banner last week, only to receive a follow-up letter Monday telling them they had not been accepted after all.
The students still could be admitted but might have to wait until April to find out, university officials said.
"Basically, someone picked up the wrong file and sent the wrong letter," UGA spokesman Tom Jackson said. "It was a mistake, and we are very, very sorry."
The students were supposed to receive merely a notice thanking them for applying.
Admissions officers have been busy fielding calls from confused students and parents.
Natalia Prada-Rey, a senior at St. Pius X Catholic High School in Atlanta, was one of the students who got an acceptance letter. She said she promptly went to a basketball game and told her friends that she got in.
"I told everybody," she said.
The second letter said the admissions staff was "very sorry if this caused you any confusion."
"They handed me my future, and then it's gone," Prada-Rey said. "I just don't understand how it could have happened."
Georgia's college scholarship program has made the state's flagship university quite competitive. On average, those accepted last year had a grade-point average of nearly 3.8 and an SAT score of 1268 out of a possible 1600.