Last fall, a month before school was to begin, 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 nursery.

The parents scrambled to find a replacement, and now worry that the replacement also may be enticed by a public school system that can offer starting salaries of $16,573 a year, compared with the $12,000 that their 52-pupil school is able to afford.

In Springfield, one nursery school discovered recently that its $200-a-year liability insurance policy with Aetna was being canceled because its agent no longer represented that insurance company.

At first, the only company the school could find to write a replacement policy was Lloyd's of London, for $2,990 a year, and that policy would not cover any claims relating to child abuse. Now, school officials are negotiating with another carrier for a $750-a-year policy to cover such possibilities as accidents.

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

All three schools are cooperatives, run by parents who find the buildings, hire the teachers and aides, buy the equipment, keep the financial records and often help out in the classrooms. In some cases, the parents even decide what lessons will be taught.

But many of the schools, which in the past catered to two-parent families with one income, are having to reevaluate their requirements and reallocate their resources. The traditional corps of volunteers is shrinking as more and more mothers find jobs outside the home. In 1980, the Metropolitan Washington Area Council of Governments reports, 56 percent of Washington area mothers with children under age 6 were employed.

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 the metropolitan area, there are 31 cooperative nurseries in Northern Virginia, 50 in the suburban Maryland counties and five in the District of Columbia.

For a nominal charge -- often between $70 to $100 a month -- parents can place their children in a high-quality educational program half a day for two or three days a week, organizers said. The fees are low because the programs are run by parents and are nonprofit.

In comparison, full-day, privately run child-care providers charge more than $150 a week, much more than many families with one income can afford, co-op 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."

Some cooperatives also have been hit hard by insurance cancellations or enormous increases in their premiums. School officials said insurance companies are reluctant to insure child-care providers because of recent publicity about sexual abuse at day-care centers in several states, including California and Florida.

Prompted by high insurance premiums imposed on some Fairfax County cooperatives, the Northern Virginia Council of Parent Participation Nursery Schools, a nonprofit group that represents 31 cooperatives, is negotiating with an insurance company to offer insurance to all members of the organization.

Hall, past president of the council, said cooperative nurseries are slowly waking up to the fact that they will be paying considerably more in the future for liability insurance, which is bound to mean higher tuition fees.

"We don't want to be stuck feeling that the only option is to pay $2,900 in premiums," she said.

Cooperatives also 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 second generation of baby-boomers begins entering school.

Of 638 teachers hired by Fairfax County this year, 270 were for elementary schools. The Montgomery County school system hired 600 new teachers this year, 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 insurance premiums 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 the cooperative, some school officials said.

Some cooperative nurseries have had to reduce their requirements for parent participation, called co-oping. Some have eliminated it altogether for a portion of the parents, and others now allow a baby sitter or grandparent to substitute at school for the parents.

The Reston Children's Center in Fairfax County, a cooperative nursery school with 300 children ranging in age from 3 months to 14 years, where 80 percent of the parents work, does not require parents to work in the classroom at all.

Instead, the parents contribute 10 hours of their time a year in activities that range from gardening to baking.

"The cooperative has begun to mean more than just a bunch of women acting as assistants in a nursery between 9 and noon," said Madeline Fried, executive director of the Reston Children's Center.

Fried also has worked for the past eight months on a contract with the National Cooperative Bank of Washington to lend technical assistance to about a dozen new and existing child-care cooperatives.

Fried said that the true meaning of a cooperative is not parental involvement in the classroom, but control over the educational program.

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 teachers' aides.

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

Another school, the Takoma Park Cooperative Nursery School, had trouble finding students this year.

Susanne Fleming, president of the cooperative, said she is afraid that the program will fold if it doesn't change. Therefore, she said, the school will begin a series of measures to make the program more amenable to working parents, such as offering a longer day and requiring parents to co-op three times a month instead of four.

Others refuse to give up on the idea of parents participating in the classroom.

Abby Owen, president of the Spring-Mar Preschool in Springfield, said that 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.

The cooperative also imposes other obligations on parents, such as helping out at the yearly fund-raising bazaar, taking children on four field trips a year, and helping with the school's upkeep.

"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."