CATHOLIC SCHOOLS are facing pressures today which promise to virtually transform them by the year 2000. As enrollments decline, as the schools lose their share of private school enrollments around the nation, and, most significantly, as Catholic schools turn increasingly to lay faculty to replace the dwindling numbers of teaching nuns and brothers, Catholic elementary and secondary education faces a financial crisis.

An addition $2 billion a year will be needed to keep pace with educational reforms, improvements in programs, and salaries for the increasing number of lay teachers, according to "Effective Catholic Schools," a report published last year by the National Center for Research in Total Catholic Education.

The drop in the number of clerics in the classroom has been precipitous. Twenty years ago, 80 percent of the teachers in Catholic elementary and secondary schools in the United States were nuns, priests or brothers. Today, as fewer and fewer young Catholics take orders, that percentage has been reversed, with lay teachers making up 80 percent of school faculties and increasing at a rate of about 2 percent a year, according to the National Catholic Educational Association.

"Historically, religious orders have heavily subsidized Catholic schools by contributing services at salaries much less than market value," according to the Effective Catholic Schools report. "Over the past 15 years, this subsidy declined nationally at a rate of about $11 million per year" mainly because of the decline in the number of religious order teachers, the report said.

While the schools need more money to pay faculty and improve their programs, the parishes themselves, particularly in urban areas, find they are unable to provide the high level of financial aid to the schools that they gave in the past. Flight to the suburbs by affluent church members in the 1960s and '70s has left many city parishes with fewer resources to educate the increasing proportion of poorer students in their parochial schools.

The student bodies of inner city schools are made up increasingly of "minorities, the poor and . . . non-Catholics," said Msgr. James F. Meyers, president of the National Catholic Educational Association. "One of my gripes is that black leadership has never favored aid (such as tuition tax credits) to Catholic schools. Yet black parents are standing in line to get their children in our schools."

THE LOWER-INCOME families in inner cities are likely to suffer most in the worsening financial pinch, said Peter B. Holland, director of curriculum and instruction in the Lynnfield, Mass., public schools. Holland, while a graduate student in the Harvard School of Education, helped author the Effective Catholic Schools report. "This is a shame because those schools offer an alternative to people who really need it." A recent study of Catholic high schools by the National Catholic Educational Association shows that about one-third of all Catholic high school students come from families with incomes under $20,000, and one-third from households with incomes between $20,000 to $30,000. (Eighty percent of all Catholic high school students are enrolled in college preparatory programs.)

Parish schools in the suburbs and those operated by religious orders continue to raise tuition because parents can afford it in most cases, "but in inner-city schools . . . (parents) are right at the margin" economically, Holland said.

Like most public education systems, Catholic schools have suffered from declining enrollment during the last two decades. And as enrollments have fallen, reflecting a sharp drop in the birth rate on the heels of the post- World War II baby boom, schools have been closed. The number of schools dropped from 13,292 in the 1965-66 academic year to 9,401 in 1983-84. Schools are continuing to shut their doors, although the 31 closings last year nationwide represented the smallest drop in one year since the 1960s.

While their enrollment has dropped, Catholic schools also have seen their share of the total private school population decline from 87 percent in 1965-66 to 63 percent in the 1980-81 school year, according to a statistical report published last year by the National Catholic Educational Association.

Schools in the Washington Archdiocese face the same conditions as other Catholic schools in the nation, according to John Convey, an associate professor of education at Catholic University and director of a study of elementary schools in the archdiocese. The purpose of the study is to develop a 15-year plan for the schools.

During the last 20 years, he said, "The archdiocese has had a surge of people moving into areas which do not have schools, for instance upper Montgomery County." There are only two Catholic elementary schools in the upper county area, at St. Martin's parish in Gaithersburg and St. Peter's in Olney.

The archdiocese has not built any new elementary schools since the mid 1960s, and the study Convey is directing will determine where new ones are needed.

While the Catholic student population has risen in the outer suburbs, enrollment has declined in parish elementary schools in some suburbs inside the Beltway and in the District of Columbia.

A high proportion -- 50 percent -- of the students in D.C. Catholic elementary schools are non-Catholic, Convey said. In one parish in the eastern part of Northwest Washington, 72 percent of the students are not Catholics, and in another in the Southeast, 62 percent are non-Catholics. The percentages of non-Catholics are much lower in the Maryland counties: 9 percent in Montgomery, 17 percent in Prince George's. Throughout the United States, non-Catholics made up 10.6 percent of all parochial school students in 1982-83, up from 2.7 percent in the 1969-70 school year, according to a statistical report issued by the NCEA.

The increasing number of lay teachers in archdiocesan elementary schools also follows the national trend, he said. Lay teachers make up about 85 percent of the faculties of Montgomery County Catholic schools, 75 percent in District schools, 83 percent in Prince George's and 71 percent in the three southern Maryland counties.

FOR THE MOST part,"Catholic parents went through the shock" of not finding nuns and priests in the classrooms in the early 1970s, when the schools first began hiring lay teachers, Meyers said. After the initial concern faded, parents accepted the changes "extremely well."

In fact, lay teachers and administrators are "an improvement. I would never want to go back to all religious faculties. Lay people add a new dimension," he said. "We expect teachers to be role models, and not all our students will (join) religious orders." Students in today's schools will be able to "see that you can be a good, religious person" without being a nun or priest.

Twenty-four of the 83 schools in the Washington Archdiocese are headed by lay principals, and in 19 the entire faculty is composed of lay teachers, according to an archdiocesan spokesman.

Sister Francis Paula, a member of the Order of St. Francis for 50 years, left her post as principal of Our Lady of Lourdes elementary school six years ago, along with the last few teaching nuns. Dan Kerns, her successor, said he and the other lay teachers were "well suited for the transition" because most of them had worked with the nuns for several years. The Franciscan sisters withdrew from the school, Kerns said, because of their declining numbers and because new sisters entering the order and the older ones who remained had more work opportunities. Once specializing in education and nursing, he added, the nuns now can go into social work, pastoral jobs in administration and other vocations.

At the time, "we had to assure parents that the quality of education and the spiritual perspectives had not changed,'" said Kerns, adding that families accepted the all-lay faculty "very well."

"We are still very much a parish school. The priests assist our teachers in religious teaching," Kerns said. The enrollment of the school rose from about 200 to 235 pupils this year.

To meet its increasing costs, the school has raised tuition by "30 to 40 percent," Kerns said. Tuition now is $1,000 for the first child in a family, half that for the second, and about a third for the third child. There is no charge for additional youngsters. The Home School Association raises from $10,000 to $12,000 a year, most of which is used for items such as playground equipment, library supplies and other "special needs that we would get along without" if necessary.

"Schools can't run on fund-raisers," but they do take a little of the financial pressure off, Kerns said. Major expenses, such as salaries and overhead, must be covered by tuition and the parish contribution to the school. He added, however, that "fund raising has become a larger component of the general financial picture" for schools in recent years.

In many areas of the country, small schools are being consolidated and others are selling buildings and land that have grown in value in commercial areas of the cities. The most recent example in the Washington area was the controversial sale last autumn of the 8.2 acre campus of Immaculata Dunblane and Immaculata Preparatory School, elementary through high school institutions, located at Tenley Circle in Northwest Washington. The school complex was the seventh sold by the Sisters of Providence since 1970. More than a third of the order's 955 members are over 70 years old, and 90 of them need skilled or assisted nursing care, according to a statement issued by the order when the sale was announced. American University bought the property for $7.6 million.

While financial problems may seem overwhelming at times, the forces that are sweeping through the Catholic education system are bringing with them some welcome changes.

"My sense is that schools will be an area in which a newer kind of Catholicism will show itself," Holland said. "Lay leadership and more of the social responsibility and social justice work the church has emphasized since Vatican II will have a chance to be played out in schools."