When 15-year-old Taylor Kiland of Arlington went to a beach in France last month, she was a little shocked to see several women in topless swimsuits.

"They're not shy, I'll put it that way," she said last week. "I guess it's like the difference between wearing a one-piece or a two-piece suit here."

The difference between French and American swimsuit styles was only one of the things Taylor and 11 other students from St. Agnes School in Alexandria had to get used to when they spent three weeks in France as part of a special exchange program. From July 1 to July 24, each girl lived with a student from L'Institut de L'Assomption in Paris. L'Assomption is a private, Roman Catholic school for girls; St. Agnes is an Episcopal girls' school. Last week, the French students arrived here to spend three weeks with their former guests.

Denise Van Swearingen, a French teacher at St. Agnes and the program's American coordinator, organized the first exchange in 1980. The purpose, she said, is to help the students learn about the language and culture of their host countries.

"They live with a family, which means it's a total immersion, not only linguistically, but culturally, emotionally," she said at a reception at St. Agnes last week for the newly arrived French students.

To participate, St. Agnes students must have completed the ninth grade and second-year French, Van Swearingen said. The French students range in age from 14 to 18 years, said Myriam d'Ussell, an administrator at L'Assomption who coordinates the French side of the program. All live in Paris and have studied English for at least three years, she added.

Taylor Kiland spent her French excursion with 15-year-old Victoire Boussac and her family. For a week, they stayed in the family's Paris flat and toured the city before heading for the coastal city of Arcachon, where they rented an apartment overlooking the Bay of Biscay on the Atlantic Ocean.

One of the things she enjoyed most in France, Taylor said, was the clothing.

"They're about six months ahead of us in fashion," she said, describing the colorful miniskirts she saw in Paris.

On the other hand, she found the French slightly behind in American-style TV programs.

"It was very hard to explain soaps to Victoire ," she said, because they are not shown on French TV. "They do have 'Dallas' but they're way behind."

For her part, Victoire already has seen a bewildering array of Washington sights, including the Washington Monument, the Lincoln and Jefferson memorials, and the Air and Space Museum.

"I think Washington is very beautiful, and I like it very much," she said.

Another French student, 16-year-old Valerie Delahaye, is staying here with Barbara Grant and her family. Delahaye said she was surprised about the legal driving age in the United States.

"The boys and girls drive at 16 years old and have a car," she said. "In France, if you're 16 years old you drive a bicycle or a motorbike." The driving age in France is 18, Van Swearingen said.

While in France, the American students discovered the difference between classroom French and the spoken language.

" The French speak a mile a minute," said Taylor.

"I didn't understand a thing they were saying in the beginning. In the end, I understood almost everything," added Helen Aman, 16 of Annandale.

The French students who arrived here last week are finding the same problem. But like their American counterparts, they have no doubt that they will get the hang of the new language.

"The American family is a good family for me. . . . I can speak English with them and learn new words," said Delahaye.