The sign outside Carlson Wagonlit Travel, on JFK Street, advertises flights to Los Angeles for $269.

But don't try to book one for Christmas Break.

A flight during the holiday season now sells for $309 at the agency, because students have already snatched up the cheapest flights at Harvard Square travel agencies.

"We're going to be busy in the next couple of weeks telling people of the high prices," said Danny Bensi, a supervisor at STA Travel, on Mt. Auburn Street, although he said that this year's prices are no different from last season's. As the weeks count down to the holidays, he said, prices rise rapidly.

"People see a $300 ticket and say 'I always fly home for $200,'" Bensi said. When they call back later and say, ''I'll take that $300 ticket,'' he tells them "that ain't happening anymore, pal." By that time the price of the ticket has risen again.

Most students seeking overseas flights for Christmas have already booked them, said Eduardo Araya, a travel agent at Carlson Wagonlit.

Now, the Square agencies are crowded with U.S. residents booking their flights home.

"At Thanksgiving, parents ask [students] if they've bought their tickets for Christmas, and probably give them the go-ahead to use the credit card," said Jasna Nightingale, a travel consultant at Council Travel on Eliot Street.