Sixteen-year-old Donna James, a Ballou High School Junior, has figured out the formula. If you separate a few items from last spring and fall, spice with some essential accessories and push up your sleeves, you are dressed to handle about anything at school this fall. "It's real bad that way," said James, obviously meaning it's For many District high school students, dressing for school suddenly means dressing up. Plenty of jeans are being worn, but many girls are wearing them with high heels and belted sweaters or jackets. Some of the guys are, too. Christopher Brown, 17, a Calvin Coolidge senior, shows off the Calvin Klein label on the rear of his jeans and has traded in his T-shirts for the alligator appliqued Izod variety.

James insists on designer jeans, too, "because they are cut straighter and fit better," she says. This year she wears them longer than before and rolled up. "Jackets and sweaters now have to be belted," she pronounces with the authority of a Vogue editor. Of course, she has belted her sweatshirt-style sweater. She's twisted and rolled her hair in a retro style that she knows to be the "It's all pretty classic, this wardrobe high school students are bound to wear this fall: sweaters, pleated skirts, pleated trousers, blazers, suits and the like, translated into the lingo of this generation. Because they were too young the last time these styles were popular and never saw their parents wear them, today's teens consider these clothese totally new.

What they do with them is certainly new. In the 1950s, the last time these classic styles were so popular, who would have thought of wearing them with cowboy boots, very much an "in" style for both sexes this fall? Patterned hose for the women is a variation on the old theme and so, for that matter, are some of the colors, particularly the plums and purples and bright color accents that are beginning to appear.

The last time pleated pants were so popular -- that dates back even before the 1950s -- they weren't worn as Elston Calhoun of Calvin Coolidge wears them. He starts with a wing collar shirt, adds a straight knit tie, tops it with a tweed wool pullover highlighted by a tiny diamond stud in the shawl collar, and tucks the sweater into his pants set off by a leather Cardin belt. On his feet, expensive wing tip shoes.

If you dress with such care (and expense) it is essential to keep a grooming kit handy. So this year Calhoun will carry a YSL canvas zip-top bag to keep his brush, cologne and wallet handy.

Skirts are bound to be part of dressing up this year. Calvin Coolidge senior Sondra Dickey, 17, this week, has always worn a lot of skirts because she finds it difficult to find pants long enough for her 5-foot-10 1/2 height. She'll still wear several skirts from last year -- after a slight shortening -- but the new skirts will be pleated, the sweaters tucked in and whole look belted. To be dressier, Dickey, who is on the Teen Board at Woodward and Lothrop and buys most of her clothes there, plans to wear a lace collar with her sweathers and buy textured and colored pantyhose.

Over at Duke Ellington School of the Arts, what you wear depends on how messy your work will be that day. The visual arts program, painting, sculpting and such, leaves clothes dirty, so when dabbling in paints or clay, jeans are the rule. But other days, Christopher Green, an 18-year-old senior, says he wears a jacket, shirt and tie. He calls it "my Ralph Lauren look," but Lauren should only be so imaginative.

Green scouts every thrift shop in town and on his most recent trips came up with his current favorites, a navy wood double breasted, ventless jacket, a tuxedo pleated-front shirt whose collar he reshaped and a cowboy string tie.But there's nothing "thrifty" about the shiny cordovan leather pointy-toed, slanted-heeled cowboy boots he bought recently.

Visual arts students Gina Brent, 15, figures when she isn't going to be messing up her clothes, she'll be ready with a very lady-like suit with velvet skirt and two-tone shoes. "If it is a nice day, and I don't have to get dirty, that's when I dress up," she says.

Maria Freeman is more philosophical about it. "If I feel it is going to be a good day, I dress up," the 17-year-old says. "If I feel good and I look good, then that makes a good day even better." For her, dressing up means a skin tight black shirt, a leopard print blouse, black beads worn knotted and painfully high-heeled black shoes. "Jean," she says, "are only for very casual days when I want to be ready for anything."

The jeans Freeman and other will wear this fall will be the skinny straight cut variety rather than the baggy ones, which are currently the rage with young girls in France and New York. But before long, baggy pants are sure to be picked up by that high school crowd that likes to be first in school with something new.

Glenna Clea, 18, an Ellington senior, is already into baggy pants, but his are of the cotton Willi Wear variety."Last year I was strictly 1940s, right down to the spats," the theater major says. His style is far more eclectic now, a collarless shirt worn as a jacket, very loose thermal underwear worn as a shirt, Willi Wear high-waisted trousers and "tramps," stylish canvas shoes.

Whatever the choices, the fashion crowd will cross the threshold back into school sporting their fancy duds and letting their imaginations loose on the next rage."