SOMETIMES, in the middle of the night, Mary Day would descend from her apartment on the top floor of the Washington School of the Ballet to the practice room below. There she would dance in the darkness, making the steps up as she went.

What with running the school, running the company, and teaching all the classes in point work, those midnight hours were her best hours for choreographing -- there in the room that was virutally her home, the windows high above eye level, grazing the ceiling, the piano jutting out from a triangular blacony in one corner.

She never wrote down what she choreographed. "The next morning I would get dancers as quickly as possible and put it on them," she says, sitting in the same room -- but by the light of morning as well as fame.

Mary Day's pupil of three years, 17-year-old Amanda McKerrow, won a gold medal in the fourth Moscow International Ballet Competition just over a week ago. Day was there with her, coaching and reassuring, and it was Day who delivered the news. McKerrow is the first student Day has taken to the Moscow competition, but not the first to other competitions. Day shepherded two former students to a similar ballet competition in Varna, Bulgaria, in 1971, and they emerged with silver and bronze medals. McKerrow follows in a line of former Washington School of Ballet students who now perform with some of the most prestigious ballet companies in the world.

It's been years of "an uphill struggle": fighting for more money, pushing the company's annual Nutcracker, brushing off the sting of mixed reviews. "We may be criticized in newspapers for not doing certain things," she says, "when it's only because we don't have the money. Oh, they say we don't have enough dancers or good dancers or good choreography."

And now, the sweet triumph at Moscow and an explosion of attention -- interviews, a backlog of phone calls for Amanda, and constant congratulations all around. "Well, I think it will make people aware of our quality," she says of the gold medal. Behind her, assembled dancers, sweating from just-finished classes, exercise at the barre. On the floor above, jumping feet hit the boards like the mufled thud of bowling balls. "Awful, isn't it?" says Day, grinning.

She sits facing the wall mirror, glancing self-consciously at her soft graying curls, fingering them into place. Her features are sharp, the eyes dark, large, almond-shaped. She won't tell her age. "I never tell," she says shaking her head. "Dancers are ageless." Mary Day studied with Lisa Gardiner, who danced with the legendary Anna Pavlova. In 1944, Day and Gardiener cofounded the Washington School of the Ballet.

She seems dazed by her success, startled to see her words of complaint, voiced after long, tiring days in a foreign country, quoted in newspapers. It's as if people were all hanging on her every whisper now. "We were terribly excited about it. When you think of the press . . ." she says, somewhat awed.

When Mary Day left for Moscow on June 9, she wasn't at all sure about McKerrow's chances for success there.

"I thought she was great, and Simon [Dow, her 25-year-old partner] was terrific," says Day. "He was a tremendous asset. When we left, I said to Amanda, 'Maybe you'll come back the Dorothy Hamill of ballet.' But I was just joking. I thought it would be nice just to dance the three rounds.

"After the first round, I knew there was something more here. We got six curtain calls for the first round. And these are real curtain calls. People are shouting and stomping and bravoing. I thought, 'Maybe she'll get something . I didn't think anyone but a Russian would get the gold."

After each round came hours of waiting for the judges to announce scores. McKerrow and Dow looked so tired, curled up on benches at the Bolshoi Theater, that Day sent them back to the hotel. She stayed for the judges' decisions. On the last night, she lay down in one of the royal boxes in the Bolshoi, all red and gold. "Spending the night in the Bolshoi," she says. "I'd always wanted to spend the night in a theater."

When the judges were ready, "the media were there -- The New York Times and NBC and The Washington Post and The Washington Star and Soviet Life . . . As soon as the announcement was made, they were all screaming at me, 'Go to the telephone! Go to the telephone !" They handed me the money to make the call. That was where that quote from me came from -- 'We got the jackpot.'"

It wasn't all luck, of course. Day knew what she was looking for. "Amanda is very good in competition. She's so sure of her technique. She doesn't get excited . . .You have to be calculating. I take whom I think can do it -- who can roll with the punches."

Says McKerrow, "I had no idea what I was going into. I had different people tell me, 'You'll be dancing on a raked [slanted] stage.' But Mary Day never got me too nervous or excited. She played down the raked stage part. She didn't make a big deal out of it. She said, 'Don't worry. Your body will adjust to it.' And I was fine.

And Day knew what she wanted McKerrow to dance. "In competition, you usually see such flamboyance and extensions and fortes," says Day. "You've seen it. They all do 'Corsaire' and 'Don Quixote.' You get tired of looking at them. We were the only 'Chopiniana,'" she says, smiling triumphantly., "It is so simple, and such a perfect example of romantic ballet. There was an old lady backstage. I don't know what she did; there were so many of them. Well, afterwards, she came sobbing to Amanda and threw her arms around her. I don't know what that was all about, but obviously she was remembering when the old ones used to do it."

The gold medal aside, Russia was full of frustrations for Mary Day -- for all coaches, she says. "They have a strange idea of protocol," says Day. "Rather a lack of protocol, as we think of it." The people backstage and the dancers were all "lovely," says Day, but the competition was full of hassles. "There was difficulty getting good seats," she says. "There were never enough programs. It was very calculated. They like people fighting over them. The McKerrows got there two hours ahead of time to get programs for everyone. The ushers wouldn't sell them. I haven't a program for the third round."

She couldn't arrange a victory dinner. ("First you have to find a place open. Then you have to take cash over ahead of time.") She aregued with the conductor of the orchestra over the tempo of one of the pieces. "There was a time he spoke very badly of us," she says. "The interpreter didn't interpret it all.

"Once a russian, always a Russian. People who were friends that I knew in the dance world were very nice to me at first. They never communicated with me after they took a look at Amanda. They wouldn't have anything to do with me." She chuckles with delight. "They didn't want anyone who wasn't Russian to win that gold."

The ballet school and the company are Mary Day's whole life. She has never married. Children? "Just 600 of them," she says. Among her alumni are Kevin McKenzie (the silver medalist that she took to Varna) and Peter Fonseca, now of the American Ballet Theatre, Virginia Johnson of the Dance Theatre of Harlem, Pat Miller and James Canfield of the Joffrey, Clint Farha of the National Ballet of the Netherlands, actress and dancer Shirley MacLaine. . . She ticks them off on her fingers.

She can't remember a time when she didn't want to be a dancer. But her teacher, Gardiner, pegged Day as a teacher. "She had many good dancers," says Day. "I just looked like I was going to be able to carry on."

She brushes off her own past performing experience: "It's not worth mentioning." But she says she was a strong dancer. "I can turn and jump," she says, pausing to correct herself: "I could turn and jump.

"In my upper teens, I was teaching. I was always teaching somebody to dance in the backyard."

Amanda McKerrow, says Day, "was an obvious talent -- a child when she came here. She was very well trained."

McKerrow began her dance studies at the age of 8 with Phyllis Blake, a former member of the Washington Ballet. "I never got the feeling that she thought I was going to become anything," said McKerrow. "Maybe it was just my age -- I was so young."

McKerrow left when she was 11 for the Metropolitan Academy of Ballet and at 14 -- three years ago -- she joined the Washington School of the Ballet.

"At the Metropolitan, they were very strict and rigid," says McKerrow. "I just decided at one point that it was time to leave. They were very insulted when I left. They didn't speak to me for a while. When I came to the Washington Ballet I learned there's not just one way to move. You can move according to what's right for your body.

"After my first summer here [at Day's school]," says McKerrow, "my parents said to her, 'Well, does Amanda have a chance to become a professional dancer?' And Mary Day said, 'she already is.' It was just the most positive thing we'd heard."

McKerow was lsited as an apprentice in the company last season. She will dance some engagements outside the company this summer, stay with the Washington Ballet this coming season, "and beyond that, I don't know," she says.

"Amanda," says Day, "is a celebrity at the moment--all of a sudden. And she's very young. Her future will be decided by all of us when the time comes. The doors are wide open."

At the Washington School of the Ballet on Wisconsin Avenue, the front door is still shattered -- the work of vandals. The company is inactive this summer. "There were no opportunitie," says Day with a shrug. In October, there will be a big benefit with Shirley McLaine acting as the mistress of ceremonies. But until there is lots more money, the company must limit itself to 10 women, six men, and 10 apprentices.

"It's all right," Day says, smiling ruefully. "If their health holds out and they don't break something."