Last fall, a month before school was to begin, the small, parent-organized Poolesville Community Preschool lost its only teacher when she took a better-paying job with the Montgomery County school system, after 13 years with the cooperative nursery.

The parents scrambled to find a replacement and were able to hire a woman with a master's degree in early childhood education.

They are quite happy with the new teacher, they said, but worry they may lose her, too, to the public school system, which can offer starting salaries of $16,573 a year, compared with the $12,000 their small, 52-pupil school is able to afford.

The Takoma Park Cooperative Nursery School, one of the oldest in Maryland, has had trouble over the past few years attracting pupils and has extended its hours and reduced its parent participation requirements in an attempt to better accommodate working and single parents.

As cooperatives, the schools are run by parents who find their own buildings, hire the teachers and aides, buy equipment, keep the books and often help out in the classroom.

The small institutions have traditionally depended on mothers who do not work outside the home, but that corps of volunteers is shrinking.

In 1980, the Metropolitan Washington Area Council of Governments has reported, an average of 56 percent of the Washington area mothers with children under age 6 were employed.

In Prince George's, 66 percent of those mothers work, the highest percentage in the area.

Nationwide, 50,000 families place their children in parent-run cooperative nursery schools, according to Natalie Hall, past president of the Northern Virginia Council of Parent Participation Nursery Schools.

In this area, there are 50 cooperative nurseries scattered among Prince George's, Montgomery, Anne Arundel and Howard counties, 31 in Northern Virginia and five in the District of Columbia.

The schools have catered in the past to families with one income and two spouses, because for a relatively low charge, often between $70 and $100 a month, the parents can place their children in a high-quality educational program for two or three half-days a week, school organizers said.

The fees are low because the programs are run by parents and are nonprofit.

In comparison, full-day, privately run child care centers charge more than $150 a week, much more than many families with one income can afford, according to advocates of cooperative nurseries.

Women who are not employed or who work only part time fear that as the number of cooperative nursery schools dwindles, they will not be able to afford to place their children in the more expensive privately run schools, the advocates said.

"There is concern that the days of the cooperative nursery school are numbered," said Elizabeth Stein, who has two small children and serves on the board of the Cedar Lane Nursery School in Bethesda.

"Our nursery school charges between $85 and $100 a month . . . and that's a big drain on our budget. . . . But if we had to pay what a conventional nursery school costs, I could not afford to send our kids to nursery school," she said.

Parents who work full time but want their children to attend a cooperative nursery school usually rely on a housekeeper or baby sitter to care for the children when they are not in school.

Parent participation in cooperatives is often far-reaching.

In addition to sitting on the board of directors of the school, parents also help out in the classrooms and are responsible for housekeeping chores and taking children on trips.

The duties are called "co-oping." In some cases, the parents even decide what lessons will be taught in the classroom.

As the divorce rate grows and more women enter the work force, and as the number of parents able to volunteer for the cooperatives declines, the nurseries here have had to reduce their requirements for co-oping.

Some have eliminated it altogether for some parents, and others now allow a baby sitter or grandparent to substitute for the parents at school.

Some cooperatives have also been hit hard by insurance cancellations or enormous premium hikes.

School officials said insurance companies are reluctant to insure child care providers because of the recent publicity about sexual abuse at day care centers in several states.

Four cooperative nursery schools in Montgomery County have lost their insurance and at least half a dozen others have had their insurance rates double or triple in the past year, said Virginia Guest, treasurer of the Montgomery County Council of Parent Participation Nursery Schools, a nonprofit group that represents 55 cooperatives, mainly in Montgomery and Prince George's counties and the District.

The state has responded to the crisis by creating a special program to help child-care providers and other hard-hit groups obtain liability insurance.

Under the voluntary program, about 50 of Maryland's largest insurance carriers have agreed to help find insurance for businesses and groups that have had trouble obtaining coverage.

However, most of the groups must pay a $100 fee for the service, and there is no gurantee they will be able to obtain insurance.

"For a little day care center that's an enormous amount," said Sandra Skolnik, executive director for the Maryland Committee for Children, a nonprofit social service organization.

In addition to higher insurance premiums, cooperative schools are beginning to feel the effects of a nationwide teacher shortage, especially at the elementary school level.

For years, public schools were not hiring many teachers, but that trend has been reversed as the children of the baby boomers begin entering school.

The Montgomery County school system this year hired 600 new teachers, including 369 for elementary schools.

"People who are willing to work for less pay get to the point where they can't afford to do that anymore," said Pat Kinney, president-elect of the Maryland Community Association for the Education of Young Children, a nonprofit organization.

Although the teacher shortage and the difficulty of obtaining reasonably priced insurance are straining the resources of many cooperative nurseries, a more taxing problem for the schools is how to serve working parents while at the same time retaining the original intention of having parents participate in the cooperative's programs, some school officials said.

The Beth El Nursery School in Potomac plans to let up to 25 percent of the parents at the school choose not to work in the classrooms next year.

Instead, they will pay slightly higher fees, and the money will be used to hire teacher aides. The school already allows housekeepers to substitute for parents.

Ellen Darr, the director and a teacher at Beth El, said the change is necessary to accommodate younger, working families who have begun attending the synagogue connected with the school.

Abby Owen, president of the Spring-Mar Preschool in Springfield, said her school will allow innovative solutions to co-oping, such as letting a grandmother alternate with a child's parents.

But the school will not permit a housekeeper or baby sitter to substitute for a parent.

"If the parents say they'll pay someone to co-op for them, we say that's not in the spirit of the co-op . . . " Owen said. "We want parents to be aware that sending a child to a cooperative school is a commitment of time."