It was a moment in the history of Chaneyville, Md., that March night when the Parisian teen-agers arrived.

The band struck up "The Marseillaise" and the home economics club broke out a buffet. The sign on the Northern High School billboard read, "Bienvenue, nos amis Francais." Underneath stood the translation, "Welcome, French friends."

The quiet, night-stillness of starlit country tobacco fields briefly erupted in a cosmopolitan, bilingual jabber.

Fresh from three days in New York City ("marvelous, beautiful, very like a country of dreams," said Veronique Granjany), the 22 students from the E'cole Montalembert just outside the French capital were suddenly plunked down in rural America.

"We knew the school would be in the countryside," said their teacher, Jacqueline Alland, "but the students were very surprised at the size of it and the size of the parking areas for the cars. We are a small country and crowded. Here you have such space and freedom."

As they explored Annapolis nine days later in stylish jeans with zippers at the ankles, the size of things American still occasionally startled the visitors. "Even American ducks are big," marveled Serge Ducruit, Alland's fiance', as the city dock's tame mallards advanced, demanding bread.

But the message the students carried was more of youthful adaptability than of unmanageable culture gaps. Barely a week after leaving one of the world's most sophisticated cities in their jet plume, the French youngsters had fitted themselves cheerfully into the automotive ebb and flow of Calvert County life, grown used to the yellow school bus that carried them almost everywhere and formed a few opinions.

"The favorite room in the American house is the bathroom," laughed Agnes Zobel.

"In the morning, Americans spend one and one half hours to fix their hair and makeup," Pascale Maree said. "We spend more time in choosing our clothes than in the bathroom, but that doesn't mean we don't wash."

The French students, 19 young women and three young men from the Catholic school in the Paris suburb of Nogent sur Marne, are staying with Calvert County families on an exchange program. They were hosts to a group from Northern High visiting Paris earlier in the year.

The American visitors brought "enormous baggage" filled with hair dryers and other beauty equipment to France, said the French teen-agers, and the American girls all wound up with the same hairdo--big curls swept back around the temples.

On the day of their Annapolis trip, the French students could not bring along their hosts, whose presence in school was required. So they were on their own, first for a tour of NASA-Goddard Space Flight Center in Greenbelt and then for a visit to the state capital.

NASA was nice, Laurent Bouvard said with a trace of Gallic disdain, "but the computers are a bit old."

Other observations:

* "Americans hide their problems. They are prudish and don't talk about their troubles at home."--Zobel.

* "Television interrupts the American family. They look but they don't talk."--Bouvard.

* American dinners are faster and less formal than French. "It's better in America because we eat when we are hungry," said Maree, dipping into a bag of Fritos. "But we are always hungry here."

* Things France does not have and needs are cookies, pancakes, crabcakes and roller-skating rinks.

More observations:

* Despite their big cars, Americans drive more slowly than the French.

* Europeans are under the false impression that American air and water are more polluted than they truly are.

* More Americans go to church than do the French.

* Unusual American customs include holding hands for prayer before meals and being quiet in front of the flag, which "shows respect and is good," Marie-Laure Lagoguez said.

* In French homes, there is one television, which is watched in the evening; in American homes, there are three or four and they and the radio can be on at any time.

* American families don't talk as much as French families, and when they do they are "cool." In France, shouting within the family is common.

But Americans work fewer hours, come home earlier and are under "less tension."

They could have gone on for hours, these teen-agers brimming with discoveries from the American stew pot of custom and style, but the yellow school bus beckoned, waiting to take them to their adoptive families in the quiet countryside.

"Au revoir," said the French, and they were off with a happy wave.