This year's Regimental Ball, an annual holiday rite steeped in tradition at all-male St. John's College High School, will mark the end of an era.

Next December, some of the uniformed cadets on the dance floor will be young women. And, for the first time in 61 years, some St. John's students will be barred from attending the most glittery school event of the year because they will not be part of the Junior Reserve Officers Training Corps.

Sagging enrollment and rising costs are forcing changes at the 139-year-old prep school on Military Road NW, one of the region's oldest private Catholic schools and the only one with an all-military curriculum.

Enrollment today is less than half that of the 1970s, when about 1,000 students attended the school each year, and administration officials say they are hard-pressed to operate on income from only 470 students paying $4,600 a year.

"Our Catholic school system was basically built on cheap labor -- nuns, brothers and priests," said the Rev. Joseph Kleinstuber, dean of admissions.

"Every time one retires, you have to replace them with a professional teacher. That means the squeeze between tuition increases and salary expenditures gets closer and closer."

So next fall the school will admit girls into every classroom, including JROTC. It will also declare optional its longstanding military requirement, which has earned St. John's cadets and their dapper uniforms a solid place in Washington folklore.

Requests for applications from boys and girls have tripled since the September announcement. Administrators say that interest underscores an 18-month study that found the school's reputation for academics and discipline a draw but its single-sex, military environment a handicap.

"Again and again, the two things we heard around the region that are keeping people away from St. John's are compulsory military and no girls," Kleinstuber said.

But some upperclassmen say they are worried about how the changes will affect the image of their school, long considered an introduction to academia for well-bred youth, its crisp dress uniform and now discarded cape seen as symbols of its distinction.

Others say nothing, including the formal ball every Christmas, will ever be quite the same on the quiet campus.

"It's a disappointment, because this school has a lot of tradition behind it," said Cadet Col. Jose Serra, 17, the school's highest-ranking cadet. "It's sad to see a lot of tradition go away just like that."

Initially there was some grumbling about sharing classes and facilities with members of the opposite sex, teachers said, but by now most cadets have accepted that idea, if less than enthusiastically.

"They are normal, healthy young males," explained athletic director and football coach John Ricca, whose son is a freshman.

What still rankles those willing to talk about it in this highly disciplined environment is the demise of the military requirement.

"Women are in the service now, so there's no reason they can't be here," said Cadet Maj. John Mullaney, 17. "But to make the military optional, it takes away from the tradition of the school."

Michael Genecki, 17, a second lieutenant from Kensington, agreed. "Gonzaga {College High School} has lots of tradition too, because it's been around for as long, if not longer," he said of his school's Capitol Hill rival, which remains restricted to young men. "The thing that sets St. John's apart is the military."

Paul Genecki, president of the school's club for fathers, said some parents have told him they too are wary.

"Parents are concerned that if the military is optional, it's going to be hard to maintain, and therefore St. John's is going to lose one of its elementary marks of identity," he said.

Several freshmen and sophomores interviewed on campus one recent morning acknowledged that without the requirement, they might be reluctant to stick with the rigorous JROTC program, which includes daily inspections, marching exercises and weapons training.

"It's just a lot of work, keeping all that brass and stuff polished," said freshman Kendric Ford, 14, of Northwest. "It's hard to fit it in every night."

But vocal dissent has been minimal. "We've been told just to accept it as is and work with it," cadet Mullaney said. "That's pretty much what we're doing."

Preparations to bring in as many as 100 more freshmen, including girls, next year have already begun. Military and athletic uniforms for girls and civilian uniforms for both sexes have been selected. Several women's restrooms must be added to the one now on campus. And a girls' locker room must be built onto the gym.

But students and administrators preparing for the Regimental Ball at the Washington Sheraton Hotel say they still have a hard time imagining what it will be like someday soon with girl cadets in uniforms escorting dates in tuxedos under the ceremonial arch of sabers, which signals the start of the gala each year.

"It sort of hurts, for all of us, because the traditions are going," said cadet Serra. "But it's much better to be part of a school that changed than of one that stopped functioning."