Prince William County School Superintendent Edward L. Kelly said yesterday he will begin his own investigation to determine whether one of his chief aides was involved in falsifying teacher evaluations in neighboring Fairfax County.

Associate Superintendent John C. Randall Jr. was principal at JEB Stuart High School in Fairfax in 1988 when the evaluation forms of three teachers there were altered to lower their performance ratings. Although Fairfax officials have not said who they believe tampered with the documents, they held Randall responsible because he was principal at the time.

Randall has declined to comment publicly, but has told Kelly that he was not involved, the superintendent said yesterday.

"We are looking into the situation ourselves," Kelly said. "John has indicated to us that he was not involved, he was not aware of it {at the time} and he did not do it."

The incident has incited critics on both sides of Fairfax's merit pay program, which is viewed as a model for other schools locally and across the country. Although merit pay was intended as a major effort to improve teacher performance, some teachers have long feared that the ratings might be influenced by the whims of administrators.

Critics in local government have predicted that so many teachers would receive high ratings that the program would be meaningless. Teachers union leaders and others have suggested that the JEB Stuart evaluations were changed to disguise the fact that so many teachers there received high ratings.

The doctored merit pay evaluation reports were discovered in June, sparking an internal investigation. An official letter was placed in Randall's file, but no further action was taken because he had retired from Fairfax schools in 1989, sources said.

Since then, Randall has become associate superintendent in Prince William, overseeing the day-to-day operations of 20 schools and evaluating their principals.

Kelly said he was not aware of the situation until a reporter's call Sunday prompted him to contact Randall.

"I've talked with the office in Fairfax, and what they've told me is that all they know is the changes occurred while John was principal," Kelly said. "There are no indications he was involved with it or knew about it."

Kelly said that unless his investigation shows otherwise, he will not take action. "Until we find some evidence that says he's guilty of something, we're going to support John Randall," Kelly said, noting that Randall had an "unblemished record" in the 25 years he worked in Fairfax schools.

Meanwhile, Fairfax teachers union representatives called for an outside probe.

"It definitely needs to go outside the system, because I don't think an unbiased and fair investigation will be done" by school officials, said Maureen Daniels, president of the 6,900-member Fairfax Education Association.

Rick Nelson, her counterpart at the smaller Fairfax County Federation of Teachers, said the case demonstrates the need for a regular outside auditor for the school system.

Fairfax school officials said their five-month investigation double-checked the evaluations of 4,000 of the county's 8,700 teachers and found no other improperly altered forms.

"I feel very comfortable with the internal investigation," said Deputy Superintendent Jay D. Jacobs. "I feel it has covered all the areas that need to be covered."

Fairfax School Board Chairman Kohann H. Whitney (Centreville) said she has no reason to distrust the staff's probe, and noted that inviting outsiders to look at confidential personnel files could be difficult legally.

In an interview Sunday, Fairfax Commonwealth's Attorney Robert F. Horan Jr. (D) said that the alterting of internal documents such as teacher evaluations generally would not be subject to criminal charges. "If they're internal or housekeeping documents and not kept in the public record, as a rule of thumb, they're not official," he said.

But Steven David Stone, a lawyer who regularly represents the Fairfax Education Association as well as teachers involved in evaluation disputes, said state statutes are not clear-cut, and he urged the School Board to request a police investigation.

"I don't think any internal staff investigation is adequate for this," he said.