When Jalna Jones, principal of Mount Vernon Elementary in Alexandria, searched her crowded school last year for a place to put a special reading program, she found only one solution -- a somewhat desperate one at that.

She padlocked two exit doors, walled off the end of a hallway and furnished the resulting cubicle with a dozen desks and a couple of chalkboards. "As public educators, we have to make do. That's not the way it should be for our children, but that's the way it is," Jones said.

Although Alexandria's student enrollment has remained stable in recent years, some of its schools are bursting at the seams, driven past capacity by a diverse student population with increasingly complex educational and social needs, teachers and administrators say.

One-time classrooms now house social workers, speech therapists, guidance counselors, and programs for English as a second language and the learning disabled. The result is that only 236 of the 384 elementary school classrooms in Alexandria are being used for regular study.

A recent facilities study predicted that if Alexandria maintained small class sizes and its panoply of special programs, the schools' current enrollment of 9,355 could rise to 10,649 and the system would run out of space long before 2000.

T.C. Williams High School, with 2,008 students, is already well over its optimum enrollment of 1,821, according to the study. Alexandria's two middle schools still have some breathing room, with a capacity for 2,400 and an enrollment of 1,935.

But the report found that many of the city's 12 elementary schools are crowded and that the system will have to find room for another 450 grammar school-aged students within five years.

For Alexandria, like many urban districts attempting to meet children's social as well as educational needs, crowding has become a side effect of well-intentioned programs. But school officials concede that good intentions may soon fall prey to fiscal and physical constraints.

Superintendent Paul W. Masem said the consultant's report will force the School Board to weigh tough decisions on whether to spend millions on new buildings, increase class sizes or cut popular programs.

"A lot of programs" could be cut if no classrooms are built to offset the predicted 14 percent rise in student enrollment over the next decade, Masem said. He added that such popular courses as the computer laboratory and all-day kindergarten could be among the casualties.

The study by Stanton Leggitt & Associates also surprised school administrators and School Board members who hoped to redistrict and restructure the city's schools by the 1991-92 school year.

Board member Judith S. Seltz has said students would be better served if the system transformed T.C. Williams into a four-year high school and created two middle schools for grades 6 through 8. But when she heard it could cost $15 million to build a wing for ninth-graders at T.C. Williams, Seltz acknowledged that middle schools would probably remain an unfulfilled dream.

"Restructuring isn't dead, but it's moribund," Seltz said.

Board members predict that tight times will dictate that grade divisions stay the same and attendance boundaries be adjusted slightly to compensate for a growing population in the city's West End.

City educators say Mount Vernon Elementary, which draws students from 17 countries and every economic tier, is one of the best examples of how contemporary educational demands are straining Alexandria's resources as never before. Renovated in 1967 to hold 1,135 students, the school is using every available square foot to serve just 802 students. The facilities study recommended that it accommodate no more than 600 students.

Mount Vernon has given up 14 of its 49 classrooms to special programs. In 1967, all but four of the teachers sat in regular classrooms. Today, the school has 35 teachers in regular classes and 30 specialists for counseling, social work, courses for the disabled, English as a second language and other programs.

To accommodate it all, the school has been forced to conduct some classes in former storage rooms.

The school psychologist holds play therapy for emotionally disturbed children in a 7-by-10-foot room that once was a closet. Three classrooms were fashioned out of what used to be the balcony of the school auditorium.

"I'm playing musical chairs trying to find space. We are just up to here," Jones said, patting the underside of her chin.

Jones and several teachers said that today's language barriers and social problems make it impossible for Mount Vernon to return to the era when 30 students sat in tidy rows learning the three R's.

Pam Miller, who teaches English as a second language, said the school has about 100 students whose first challenge is simply communicating with the teacher and other children. Ten of the 26 students in one of Mount Vernon's kindergarten classes do not speak English.

"I'll have students with five different languages from five different countries" in a single class, said Miller. She added that it was not long ago when there was so little demand for classes in English as a second language that she traveled between five schools teaching language part time.

Lilmar Taylor-Williams, a fourth-grade teacher, said one of her profession's toughest hurdles is to reach the children whose lives have been disrupted by urban pressures and broken families. When she began her career in the early 1960s, "the kids who were bad were the ones who didn't do their homework," Taylor-Williams said.

Today many students have no one to go home to after school because both parents work just to feed and house their families, she said, adding that some children are left behind because their parents have turned to drugs. "Last year I had parents who were in jail. That impacts on the child," she said.

Mount Vernon this year instituted a program to help children learn how to interact with others. Students in all grades spend a few minutes each week practicing how to communicate "without putting up your dukes," Jones said.

Last week, teachers led children in saying "Excuse me," "I'm sorry" and "Thank you" to other students.

Students are encouraged to practice outside class, earning courtesy points for "appropriate" behavior.

Jones and Mount Vernon teachers said the school's tight quarters make their jobs difficult. They added, however, that without its programs and specialists, their jobs would border on the impossible, and children would pay the price.

The social workers, computer labs and other resources are critical "if we're going to educate our students and meet our mission," Jones said. ". . . I just can't think we would cut programs. Surely we care more about our children."