This weekend, while most Northern Virginia 16-year-olds will be contemplating snow and second semesters, Betsy Rapp will be churning through the water of Independence Park in Rosario, Argentina.

Rapp, a junior at Edison High in Alexandria and one of the nation's top women butterfly swimmers, is among 10 Americans selected by the Amatuer Athletic Union to compete in the Red and White Sports Games today and Sunday.

In a sport in which toddlers set world records and collect Olympic medals. Betsy Rapp is nothing short of remarkable.

It is not only that her swimming is remarkable, which it is: she came within two-tenths of a second of setting an American record in the 100-yard butterfly event; she currently holds the country's record for 100 meters.

What is most remarkable, perhaps even incredible, is that she has accomplished those things only three years after she began training seriously at Alexandria's Starlit Aquatic Club.

"Usually it takes five years or longer to develop a swimmer to the point where Betsy's at right now," said her coach, Holder Dietze, who supervises the Spartan training Rapp and 500 other swimmers go through each day in an effort to become champions.

"But, you see, Betsy's not just an average swimmer," he added. "Sure, she's got all the physical abilities - she's awfully strong and has great flexibility, but there's something else that she has that only great ones have . Pure desire."

That desire, said Dietze, allowed her to place fourth in the 100 meters in last year's Olympic trials at Long Beach, Calif., good enough to become the American team's first butterfly alternate but not good enough to make the trip to Montreal. That desire was also there when she recorded a 1:02.4 timing in the 100-meter butterfly, the 12th-best in the world in 1976.

Her time of 2:18.1 in the 200 meters was 24th-best in the world in 1976, good enough for eighth in the Olymbic trials and she was foiurth in the 200-yard event in the 1976 indoor national championships at Long Beach.

According to AAU officials in Indiannapolis, she was selected for the Agentina trip on the strength of her performance in the AAU outdoor national championships last August, in which she was third inm the 100 meters with a 1:03.4 timing and eighth in the 200 meters with a time of 2:18.7.

The trip to Argentina, which replaced a trip to Japan that was postponed due to financial difficulties, is the second international sojourn for Rapp. Last year, although placing second in the 100-meter butterfly at the Tilt International at Antibes, France, she set an American "short course" mark of 1:03.2 which still stands.

Over Thanksgiving weekend, she came within two-tenths of a second of another American record taking the 100-yard butterfly at the Potomac Valley meet in a time of 57.2.

"The amazing thing about Betsy is that there are plenty of people around that can beat her in practice." said Dietze. "But she must be what they call a money player. When they start the race for real, she's in there to beat everyone."

Dietze says he noticed that quality three years ago, in September, 1973, when Rapp joined Starlit after swimming for a year in a recreational program at West Point. N.Y., where her father, Lt. Col. Edward Rapp, was then stationed.

"My gosh, she wanted to work when she came here," he remembered. "Teaching her was and still is a genuine pleasure."

Still, last year, when she could have gone to the Olympics in Montreal by simply beating out one more swimmer in the qualifying meet, she did not.

"To Betsy's advantage, she realizes that there's a lot more to life than swimming," said Dietze. "Last year, she simply decided that she's in so much better shape"), belonging to the schoold's National Honor and Mathematics honor societies, and bringing home a report card that only rarely shows anything less than straight as.

And, of course, there's twice-a-day practices, 1 1/2 hours each morning before school, two hours after school, and additional practices on Saturday and Sunday.

Despite the rigors of the practices and the long hours put in each day, Rapp is only now beginning her ascent toward the elite of the swimming community.With her, Starlit last year was the 16th-ranked AAU swim club in the country and she will help them out for at least one more year.

After that, she will become a Part-time performer for Starlit, swimming three in the summer after competing for one of the several dozen schools that already have offered scholarships.

"That's when you'll see Betsy really begin to develop," said Dietze. "They'll put her on a weight program, which we've avoiding doing and she'll really improv her speed."

Until then, Dietze has a schedule planned for his 5-foot-10 star that will bring her further national exposure starting in the spring with the indoor national championships at Canton, Ohio, and the outdoor AAU championship at Mission Viejo, Calif. And, in hopes of getting her aboard a trip to Europe to compete against the feaared East German and Soviet Union teams, he will have her enter the Pan American games next fall.

Yet, with all those plans, Dietze says Rapp has several more years of developing her talents before she reaches the pinnacle of her sport.

"What I'd like to see is her hit her peak by the time she's 20," said Dietze. "That'll be 1980 and that, of course, that's the enxt Olympic years.'