At Annandale High School, they were the ultimate symbol of senior slump: pajamas.

Checkered flannel or cloud-patterned cotton, it didn't really matter. Even little bunnies could exude cool -- and rebellion.

"It's the culmination of four years of hard work," said Anteneh Addisu, 17, the senior class president. "We just want to relax. We were going to wear our pajamas till we got an acceptance letter."

But Annandale's principal didn't find the fashion movement, known as "Pajamas Till College," so fitting. This year, Donald Clausen got on the public address system and tersely told seniors that he wanted the slack to end and the slacks to return. "Sometimes you have to state the obvious, but you shouldn't have to," he told them. "Don't wear pajamas."

That set off a fiery debate at the high school, one that appears to be raging nationwide. Just what is appropriate dress for school? And, as Addisu still asks, just what are pajamas? After the principal's edict, Addisu and several friends took marking pens to pairs of sweat pants and wrote "pajamas" and "Seniors 2003" all over the legs.

Taking the lead from their parents, who increasingly dress down for work, teenagers are showing up at school in their pajamas. Some say they simply desire comfort; others claim to be pulling so many all-nighters that they don't have time to change -- let alone shower -- in the morning. In response, pajamas have been banned in several school districts across the country, from Cherokee County, Ga., to Charles County, Md. Other school systems in the Washington region have considered the issue as dress codes come up for review.

The trend, according to teenagers, is to wear the pajama bottoms with a matching T-shirt. Flip-flops or fuzzy slippers often complete the look. With teenagers spending more than $70 billion annually on clothes and shoes, market analysts say such stores as the Gap, Old Navy, Aeropostale and American Eagle Outfitters are making more room for what they term "dormwear."

"It's all about this casualization," said Brian J. Tunick, a retail analyst who tracks teenage buying. "I think the teen out there is looking for newness anywhere they can find it. This year, it seems to be sweat pants and loungewear."

Some teenagers say they wear the pajamas just to school. Heather Austin, 17, of Springfield, chooses hers according to the calendar or the season.

"On Halloween, I wore ones with little candy corn," the Annandale senior said. "The only difference between sweat pants and pajamas is that pajamas have a design on them. It's a technicality."

Administrators, however, say they know pajamas when they see them. And most don't like what they see.

"School is a place you go to conduct business, and your attire should reflect that," said Katie O'Malley-Simpson, spokeswoman for Charles County schools.

She said the county's dress code was recently revised to exclude "sleepwear and sleepwear-type clothing."

Officials for District schools, as well as those in Montgomery and Loudoun counties, say they try to keep their dress policies flexible so that principals can develop guidelines specific to each school. This week, the Fairfax County School Board is scheduled to review and vote on a dress code that more clearly spells out expectations. It would specifically ban showing cleavage, midriffs or undergarments.

"Clothing should fit, be neat and clean, and conform to standards of safety, good taste and decency," the dress code states.

After weeks of discussion, a committee of School Board members, administrators and students decided to leave the pajama issue untouched, allowing principals to decide.

"Faddish kinds of clothing might appear at different schools," said Mary Shaughnessy, Fairfax's director of student services.

"Our overriding thing is that students are expected to dress appropriately. There's certainly a lot of discretion for principals to build into it," Shaughnessy said.

As the case of pajamas shows, among finicky teenagers, clothing trends vary widely by district. At Severna Park High School in Anne Arundel, Victoria Boggiano, 15, said that wearing pajamas to school was popular years ago but is dated now. "That was so back in the sixth grade," she said.

Farther south, at Yorktown High School in Arlington, Kayla Scheiner, 18, said that pajamas haven't caught on yet -- except during "spirit days," when students dress up in costumes or colors for a big game or pep rally. "Pajama Day is the most popular spirit day," said Scheiner, a senior.

The outrage over the pajama ban at Annandale is slowly dying down, students say. As the weather has gotten warmer, they wonder whether it might disappear entirely, to be replaced by one concerning skimpier clothing.

As senior Elaine Filadelfo, 17, put it, "Pajama pants cover up a lot more than other articles of clothing people would wear."

Heather Austin, left, Jessica Gray, Kathryn Hoey, Elaine Filadelfo, and Gina Sobel sport loungewear they favored until Annandale High's ban.