In these changing times when so many men's preparatory schools are going co-ed, Madeira School in Greenway, Va., is remaining relentlenly all-female with the largest enrollment in its 71-year history.

The 320 young women who currently are spending their high-school years at Madeira are facing new challenges that will fit them for a world dominated by the new independence for women.

And the new headmistress, Jean Struven Harris of New York, is all for Madeira students learning to share a man's world, both in business and in the home.

Harris, mother of two grown sons and divorced 12 years ago, will concentrate on training the young women at Madeira, ages 13 to 18, to be leaders in business and the sciences and also happy wives and mothers.

Harris, blond and slim and much more youthful-looking than her 54 years, has come from a background of industry and private schools.

For the last two years she has been sales administration manager of the Allied Maintenance Corp. of New York. She was the first woman to ever hold such a job at the company which provides maintenance at Madison Square Garden, Yankee Stadium, for airlines and museums and the State Department here with 19,000 employees around the country. She said male workers eyed her questioningly at first, but finally accepted her. A woman has been chosen to succeed her.

She has written magazine articles on the single-sex secondary school and is happy that Madeira has crossed that bridge and decided not to admit young men. She thinks that in an all-female atmosphere young women have a better chance to learn the responsibilities for their own lives.

"One of the main things that young women learn at Madeira is that they are responsible for the quality of their own lives," says Harris. "In an all-girl school, young women do not have to worry about what the boys are thinking. They concentrate on their own development."

She feels it is beside the point that certain men's preparatory schools --such as Exeter, St. Paul's and Andover --that women get a better quality of education when they learn by themselves.

"The all-girls school provides the opportunity for what the women's liberation movement is all about," she says. "They need to learn leadership.

"Women have a lot to learn, and they must learn how to add quality to their lives and not settle for mere secretarial jobs."

She wants to see women become "doctors, lawyers and merchant chiefs."

She referred to the late Lucy Madeira, who founded the school in 1908 and who said that "life is short and sweet and a woman should plan her life as well as a man."

Harris feels that the idea of most young women today is still to get married and at the same time to make plans for her intellectual development as a man does.

Harris credits the departing headmistress at Madeira, Barbara Keyser, with originating many of Madeira's most unusual features. Foremost is the co-curriculum program in which every student works one day a week, usually on Wednesdays, off-campus. Of next fall's total enrollment, some 100 will be day students from the Washington area.

Keyser felt that the school was lucky to be so close to Washington and ought to take advantage of the contacts there, so every Wednesday, school buses carry the girls to town. Freshmen and sophomores work in the community service field -- 11 girls will work in the laboratories of Children's Hospital when the fall term begins. Juniors work as interns on capitol Hill. And seniors work in the field of their career interests.

Harris has been in the teaching field for more than 20 years, working in everything from nursery school and the first grade to the final year of the college preparatory school to overall director.

she is a graduate of Smith College, magna cum laude, with a Phi Beta Kappa key. She has an M.A. in education from Wayne State University. Her association with independent schools includes Grosse Point University School and Springside School in Philadelphia where she served five years as director of the middle school.Before she went into business in New York, she headed the Thomas School in Rowayton, Conn, for four years.

Now, after a shakedown year at Madeira, she will shape her own plans for changes. At the moment, she is trying to settle in the piles of books she brought with her and make some decorative changes in the Madeira buildings and in her own home on campus. She hopes to bring more faculty to live on campus so that students can meet teachers in an informal basis.