Corsages are bred to last until dawn, and the valet-pressed tuxedo shirts were still crisp, but at 5 a.m. yesterday, Bo Kimbrough was beginning to fade.

More than 12 hours after struggling with the unfamiliar components of black-tie formal dress, Kimbrough was still immaculate. But at last, over quiche and sausage, Kimbrough, his date, Natalie Buford, and the three other Fairfax High school couples who have pledged Prom Night to each other, feel the pale finality of first light.

"I guess you just don't want it to end," Buford said as her mother finished cooking breakfast in the kitchen of their Oakton home. "Mom? What time's sunrise?"

Acid rock may give way to New Wave and peasant India prints to Ralph Lauren, but prom night remains a round-the-clock ritual of growing up and for many teen-agers the social and sentimental apex of high school.

So important is this night that one night just doesn't seem long enough, and when the last dance is done the event continues with rounds of breakfasts, parties, and trips to the beach until finally sleep cannot be held off any longer. They want it somehow to last through a whole weekend, through an eternity of end-of-school exhilaration.

For Buford, Kimbrough and their friends, the prom--nominally, a 9-1 dance at the Ramada Inn in Alexandria's Old Town--actually began about 4 p.m. Saturday with a full-dress photo session. Then dinner at Geranio in Old Town, dancing and suite parties at the Ramada and finally, a silver-and-crystal quiche breakfast for eight prepared by her mother and aunt in Oakton.

"I like having them here," said Shirley Buford, who made pre-dawn breakfast last year, too. "They'd have to eat breakfast anyway, so they'd go to some all-night restaurant, and I'd rather have them here. I can see them safe and sound.

"Besides, they like to be treated once in a while. I think it's fun for them to have the china and the silver and all."

At almost every high school the all-night prom has become tradition. For Mike Koplan, a Woodson High junior who donned his first tux Friday, the prom began about 5:30 p.m. with a deli spread at a friend's home. Next, dinner (filet mignon) with about 20 other Woodson students at La Guinguette in Merrifield, dancing till 1 at the Sheraton International in Reston, and finally, "two or three" champagne breakfast parties at the homes of other students.

"It's great--everybody's together," says Koplan, and to many of the prom-nighters the all-night partying is proof that their fraternity will last forever.

"It's like those phone commercials. We'll always talk about going to prom," said a stunning young woman in a 7-Eleven parking lot whose date was buying ice for their California champagne early yesterday morning. "It's like seeing your friends in a movie, with beautiful clothes and lights."

Prom night is also the seniors' last chance to make a big splash. West Springfield High senior Chris Sabec and his friend Doug Leland spent more than $500 between them "to go as classy as we could."

Sabec and Leland hired a $40-an-hour limo complete with champagne buckets, TV, stereo, telephone and an electronic bar that automatically dispensed premeasured spirits. They rented top hats and tails and ordered a catered dinner for four--stuffed lobster, asparagus wrapped in prosciutto with hollandaise--at Sabec's home.

"Our parents thought we were crazy. They said we were going overboard," said Sabec. "But it's the last blast. The girls told us they never felt so spoiled."

The nature of the excesses of prom night to some extent reflect the economy. Although the trend in recent years has been toward elaborate pre- and post-prom parties, almost like the stirrup cup-to-dinner hunt weekends of the last century, money belts seem just a tad tighter this season.

Staffers at Ridgewell's Caterers, who have provided steak and croissant breakfasts in other years, say students' families are hosting the functions themselves this year. And one travel agent, who had a handful of inquiries about chartering all-night train expeditions to New York and the like, said quoting prices brought the conversations to a quick end.

The fluctuations of social convention leave their marks as well. In recent years, hotels routinely rented out rooms to students to host prom-night parties, but this year--often, at the request of high school principals, hotels said--they would not. And some that did, such as the Old Town Ramada, had police administering sobriety tests and preventing students from passing back and forth from the ballroom to the hospitality suites.

But parents' perennial fears of illicit alcohol and adolescent hanky-panky seem almost outmoded nowadays. There is a straightforward, even strait-laced simplicity about these students, who say a fancy-dress ball is to be seen at, not swigged away in the back room.

"That's just stupid," said one Fairfax High senior. "Besides, who's going to take a chance on ruining a $200 dress?"

The beach is still the final call on many prom-night itineraries. Some partyers "crash" for a few hours' sleep before heading to Delaware; others take a week or 10 days' sun between final exams and baccalaureate. At least a few still talk of driving to Ocean City for their post-prom breakfasts just to see the sun break over the water.

Others look for freedom in their own back yards. "We were broke," said Herndon High senior Claudia Testa, "so we decided to camp out." Four couples, who began their prom night a few weeks ago with dinner at the Rusty Scupper in Tysons Corner, spent the last part of the night in tents, in the pasture behind Dan Horner's home in Great Falls.

The things that change about a prom are the little things, dress styles and dance music. This year the dresses were full again, the shoulders bare, the shoes not too. Coats with tails were popular, and the shirts more sedate--plain whites and pale blues.

Roses are back, and the musky gardenias are out. Disco is still dead, and Michael Jackson has set a new standard in stylized soul dancing.

But romance lives on, and poignantly. In the rainy sheen of a hotel parking lot, a white rose huddles in the gutter grate, broken in a moment's argument. In a hotel powder room, crumpled tissues bear the eternal kiss of bright lipstick.

And sunrise remains the real denoument, when for the last time, the teen-agers see each other and themselves brightly dressed and beautiful with expectations, and in the prussian blue of the false dawn, life after high school seems infinite indeed