Kathleen G. Johnson, the acting head-mistress of Madeira School, paused during lunch at the school cafeteria yesterday to recall what Lucy Madeira, the school's founder and namesake, said about crises.

The first saying was "Keep calm at the very center of your being" and the second, Johnson remembered, was "Function in disaster. Finish in style."

"Of course, Miss Madeira didn't expect a disaster like this," Johnson said. "But she expected you to cope, regardless, and we are -- we are sending out report cards today."

The 74-year-old school had been stunned two days ago by word that Jean S. Harris, its headmistress, had been charged with second-degree murder in New York in the shooting of Dr. Herman Tarnower, creator of the popular "Scarsdale Diet."

Yesterday the private girls high school in McLean was, as one teacher put it, "carrying on."

Most of its 324 students were on spring vacation yesterday but about 40 students had remained in the area, working on Capitol Hill internships, Johnson said. Administrators were on the campus yesterday attempting to cull through the applicants for the fall classes.

Although the costs for the school are relatively high -- $7,000 for boarding students and $4,200 a year for day students -- Madeira is not suffering from a lack of applicants. This coming year Madeira will recive about five applicants for every espace it has, a measure of the strong following the school has developed since it was founded in 1960 a row house near Dupont Circle.

In 1931 Madeira moved to a 381-acre campus about 12 miles from downtown Washington on a bluff overlooking the Potomac.

Most of its buildings are set so far back that no one can see them from Georgetown Pike. Outside its white fence there is no sign to tell those who don't already know where they are.

Near the main gate are a larger stable and an indoor ring for riding horses, which school officials say is one of the largest in the East. There are tree-shaped walkways, Georgian-style brick buildings, a drawing room with Chippendale, furniture, and magnificent views of the river rushing below.

There also is a library with 20,000 books, a modernistic science building that won a series of architectural awards, and a full-time faculty of 32, plus nine part-timers, for its students, who are enrolled in grades 9 to 12.

Last year five of its 76 seniors were National Merit Scholarship semifinalists and all but one of the graduates went on to college. Three each went to Princeton, Yale and Brown, and two each to Wellesley and Vassar.

Harris, who became headmistress of Madeira in July 1977 and lived on campus in a red-brick house called "The Hill," toughened its graduation requirements, while keeping its discipline firm.

"She was a charming woman," the parent of one student remarked. "But she was very strict. She didn't put up with anything and we found that very attractive."

Harris held to most of the school's strong traditions, but put new emphasis on recruiting, producing an upsurge in both the number and quality of applicants.

"A lot of [private boarding] schools folded in the late 1960s and early 1970s," said Katharine Woods, Madeir's admissions director, "because there was a supply and demand problem. Madeira turned things around several yeard ago. We're in healthy shape now."

With its high tuition and the riding ring near its front gate, Madeira has acquired a reputation as a "horsey sort of social school," one student remarked. "But that's not so. The horses are here but most of the girls don't ride them now."

About two-thirds of its students are boarders, living in comfortable double rooms but hardly in splendor. Not only do students clean their own rooms, but they rake leaves on the school grounds, Johnson said, and serve meals to each other at dinner.

The students come from 30 states and 19 foreign countries, though about 60 percent are from Virginia, Maryland, and the District of Columbia. a

Current students include the daughters of Sen. Malcolm Wallop (R-Wyo), ABC television reporter Sam Donaldson and commentator Eric Sevareid as well as the granddaughter of the late former vice president Nelson Rockefeller.

Among its alummae are Katharine Graham, chairman of The Washington Post Co. and Ann Swift, a U.S. foreign service officer now being held hostage in Iran.

Although no blacks were admitted until the mid-1960s, officials said Madeira now has 12 black students and is seeking more.

About one Madeira student in 10 receives financial aid.

"We'd like to have more scholarship students," another official remarked. "But we can't afford it."

Although it has an endowment of about $2 million, the fund has hardly grown in recent years despite rapid inflation. To help balance its current budget, the school runs a summmer camp, rents-rehearsel space and a dormitory to performers at Wolf Trap, and operates an outdoor adventure program on its rugged hillsides and wooded grounds.

For its own students the academic grind is tough -- from 8 a.m. to 3:30 p.m. four days a week, with required extracurriculr activities running until 4:30 p.m. Most classes include 10 to 12 students, and there is a heavy dose of mathematics -- somewhat rare for girls' schools.

On Wednesdays all sopohomores, juniors and seniors are away from campus, working as unpaid interns in congressional offices, nonprofit organizations, and museums or as volunteers in hospitals, day-care centers, schools and nursing homes.

"Madeira offers very adequate preparation if you are interested in marrying a nineteenth century English Lord," declares "The Insider's Guide to Prep Schools," pubished last year by the Yale Daily News. But the Guide adds: "The school also tries to prepare its students for life in twentieth-centruy America . . . You can get a good education here . . . Most Madeira students love their school, and it's easy to see why."