A judge ruled yesterday that Fairfax County may not force the transfer of a 16-year-old retarded McLean girl from a residential institution in Maryland to a day school for the retarded in the county.

Fairfax Circuit Judge William E. Plummer ruled that the County School Board faled to prove its contention that the daytime facility was appropriate for the girl, Paula Jane Hurley. "Each child is unique," he declared, " and this child has done very well in residential placement."

The case provided a clash of competing philosophies of special education. Since Monday, a parade of witnesses -- many of them experts -- had argued whether Paula, a severaly retarded, nonspeaking girl with a mental age of about 3, could receive an "appropriate" education, as state and federal law requires, in a six-hour-a-day institution.

Although the county's attorney denied it, several county witnesses said immediately after the decision that the result of the school board's four-year-old case against Paul and Rae Hurley, Paula's parents might encourage other parents to place their retarded or multiply handicapped children anywhere they pleased, including residential institutions outside Fairfax County.

Ann W Mische, the board's attorney, deined that the decision set any precedent. She stressed that the judge made his decision on Paula Hurley's individual needs. Mische further insisted that the county had pressed its case since 1977 -- at first in administrative appeals and ultimately incircuit court -- only because the school board believed it could offer Paula Hurley an "appropriate" education. Cost was not an issue. The county pays about $8,700 a year for Paula's education and that figure would be about the same at the day school, according to papers filed in the case.

Mische said after the decision she doubted that the school board would appeal.

The parents and their witnesses argued that Paula should remain at the Benedictine School in Ridgely, Md., where she enrolled in 1978, shortly after the Hurleys moved to Fairfax County.

They maintained that Paula had made significant progress at Benedictine especially in the areas of sign language and control of bodily functions. Witnesses from Benedictine further stressed that Paula's parents were unable to provide the sort of consistent, round-the-clock attention and expertise they said Paula needs.

The county held that a residential school was inappropriate for Paula because it had not taught her to apply the skills she learned to daily life in the outside world. They said the Kilmer Center, a public day school for the retarded in the Tyson's Corner area, would teach her to generalize her skills outside school hours.

While the parents cited recent incidents at home and said there was a danger of Paula regressing if she were removed from the residential school, the school board argued that the incidents had occurred because Paula had not learned to function adequately outside what it described as the overstructured, restrictive setting of the Benedictine school.

The judge's ruling came abruptly, after the county had presented its final argument, but before the parents' lawyer had presented theirs. The Hurleys said they wre not surprised at the verdict, and added that they were grateful the trial was over.

"It has been awful," said Rae Hurley, rubbing away tears. "It's been such an ordeal."

Meanwhile, their lawyer, Steven Stone, accused the school board of losing sight of the public interest in pursuing the case. He called for an investigation of school board personnel involved in the case, and asserted that the county had spent at least $20,000 in legal and related funds to prosecute the case.

Paul Hurley declined to say how much the case had cost him, but he did say that it had posed grave financial problems for his family.