The Loudoun County School Board must replace former Superintendent David N. Thomas within 120 days, or face the prospect of having a successor named by the state Board of Education, a state education official said this week.

According to state law, school officials have 60 days to begin a search and the option to ask for one 60-day extension, said Frank Barham, director of the Virginia State School Boards Association.

"There's a feeling you need your {superintendent} in place," said Barham. He said the law is intended to help keep school systems from foundering, as they might without a school chief.

Thomas, 56, was fired last week by Loudoun's School Board after less than three years as head of the system. The board said his management approach did not suit the growing system. Several school officials said his aggressive style had hurt the morale of teachers and administrators.

The state deadline means Loudoun's board will have to begin looking for a superintendent much sooner than planned. Several board members said they had hoped to complete difficult budget deliberations this spring before starting a search.

"I don't want to rush into anything. I want to conduct a proper search," said board member Barbara B. D'Elia (Dulles). "The next big process is the budget . . . . We have to really determine what we want" in a superintendent.

Thomas, a Harvard-trained educator, will continue to receive his $97,000-a-year salary, the highest of any county government official. School Board Chairman C. Carroll Laycock Jr. (Blue Ridge) said the school system will likely have to pay Thomas as much as $210,000 for the remaining 27 months of his contract.

Laycock defended the board's decision and the expense it means for the county, saying morale had become a problem. When divided over two budget years, and in the context of the system's overall spending, the total "is really a small amount, a relatively small amount of money," Laycock said.

In a vague statement issued after Thomas's dismissal on Jan. 14, the board said Thomas lacked the type of leadership needed to run the 14,600-student school system. The statement did not provide details about what led up to his dismissal.

School officials, who did not want to be identified, have said in interviews since the firing that the board action culminated two years of frustration over Thomas's management style. He has been described by his critics as unapproachable, overly aggressive and even abrasive.

During the past two years, Thomas was asked repeatedly in closed-door meetings to tone down his approach, one official said. The School Board was especially concerned about offending the county Board of Supervisors, which decides how much money the school system gets each year.

The board's frustration with Thomas began to peak last year, when Thomas pushed the supervisors for teacher raises and irritated some of them with his aggressiveness, one key school official said.

"The superintendent must realize he must deal with the School Board, and the School Board must then go to the Board of Supervisors," the official said. "That was one of his problems."

But the School Board's unease with Thomas became acute late last summer after Dianne P. Mero, a new but popular administrator, resigned suddenly without public explanation, the official said. That was the beginning of Thomas's demise, the official said.

Mero had been in charge of the county's middle schools and high schools.

"It was one of the major things," the official said, adding that Mero left out of frustration. "I think that sent a shock wave, not only in the board but throughout the system."

Thomas did not want to discuss Mero's resignation. Mero could not be reached for comment.

The School Board decided to fire Thomas after he excluded Assistant Superintendent Edgar Hatrick III from a staff meeting this month on the budget. Thomas said last week that Hatrick, a Loudoun educator since 1967 and a candidate in 1988 for superintendent, was trying to undercut him politically.

Six members of the School Board disagreed with Thomas's decision to exclude Hatrick. "These were not just routine meetings. These were critical meetings," the official said. "When you have a key person, why do you set them aside? We said, 'Hey, this is a key person.' "

D'Elia, one of two board members who voted to keep Thomas, said this week that she continues to support him and his ideas about education. She said the board should not have fired him, particularly when the school system faces unprecedented cuts in state and county funds. D'Elia said parents have been calling her in support of Thomas.

"I have been fielding a lot of calls from very irate parents," she said. "I think Dr. Thomas had a lot of support . . . . I don't think the problems were insurmountable. I think the problems could have been solved."

The School Board does not have a list of candidates for superintendent.

Laycock said he wants to hire someone who is familiar with the Virginia school system. Virginia differs from other states, he said, because its school boards are not elected and don't have the power to tax.

"I think it's important that we have someone who understands how Virginia politics works," he said. "Virginia is different."

Hatrick, a candidate for the superintendent's job three years ago, said he likely would seek the job again. "Yes, I probably will," he said.

On Monday, Thomas said he had not made plans yet about his future.