The Loudoun County School Board has fired its superintendent of schools, less than three years after he was hired to manage the growing suburban school system.

In a move that took teachers, administrators and parents by surprise, David N. Thomas, who left a top job in the Montgomery County school system in 1988 to come to Loudoun, was fired Monday on a 6 to 2 vote.

"The leadership style was not in the best interest of Loudoun in the long term," School Board Chairman C. Carroll Laycock Jr. (Blue Ridge) said yesterday. "That really is the reason."

School administration sources said that Thomas, 56, had an aggressive, often abrasive approach that undercut morale among teachers and administrators in the 14,600-student system.

Thomas also has had a sometimes contentious relationship with Loudoun's Board of Supervisors. Tensions rose last year during talks over teacher pay raises. Thomas pushed tirelessly for a proposed $35 million high school that some supervisors say the county cannot afford.

One school official said the School Board feared that Thomas would hurt relations with the Board of Supervisors at a time when, because of state cutbacks, the school system would have to rely increasingly on local tax revenue.

"There have been occasions when he rubbed people the wrong way," said the official, who asked not to be identified. "We're going to need that money. We're going to all have to be good ambassadors."

Kathleen Franklin, president of the Loudoun Education Association, said many teachers were relieved yesterday by Thomas's departure, despite his support for higher teacher pay. She said he had an overbearing style that intimidated teachers, administrators and clerical staff members.

"A lot of people think it was a good decision," she said. "I was surprised it happened at this time."

Thomas said he feels good about his tenure in Loudoun. He acknowledged that he has sometimes been irritable in the last 18 months, in part because of the death of his 16-year-old son in a traffic accident.

"When something like that happens, as anyone who's experienced it knows, there's a lot of free-floating anger," Thomas said. "I was short-tempered at times."

The firing of Thomas comes as other school superintendents in the region and across the nation face troubles of their own. D.C. Superintendent Andrew E. Jenkins was fired in November; school chiefs in Prince George's and Montgomery counties have announced they will be leaving their jobs, and Superintendent Paul W. Masem is under fire politically in Alexandria.

Thomas, a Harvard-trained educator, said he was uncertain about his next step. "Perhaps I will seek employment in the local business community or, if I can stomach it, seek another position as a superintendent of schools," he said.

Thomas school administration sources said that the board decided to fire him after he had a conflict with an assistant superintendent, Edgar B. Hatrick III, a longtime employee of the school system who applied for the superintindent's job in 1988.

The School Board met privately with Thomas for several hours on Jan. 8 and ordered him to resign or be fired on Monday. Thomas refused to resign.

Hatrick did not return several telephone calls yesterday.

Because Thomas has more than two years remaining on his four-year contract, he will continue to receive his $97,000-a-year salary, said Laycock, pending the outcome of negotiations with the School Board.

Associate Superintendent Harry Bibb took over Thomas's duties yesterday. Laycock said the board will not begin a search for a new superintendent immediately.

Board members Joyce S. Rocks (Catoctin) and Barbara B. D'Elia (Dulles), who voted to keep Thomas, blasted the board's decision yesterday, saying that Thomas had improved Loudoun's system by trying to make it competitive with schools in Fairfax and other suburban districts. And they criticized the timing of his firing, a week before the announcement of next year's school budget.

"I cannot fault him in any way in his education vision," Rocks said. "I think this is a terrible time to make this sort of decision."

Maureen Dabkowski, president of the Loudoun Education Alliance of Parents, said Thomas was improving Loudoun schools.

"I'm not happy about it," Dabkowski said. "I saw a lot of good come out of his administration."

A one-time physics teacher and a graduate of West Point and Harvard, Thomas was only the fourth superintendent since the school district was established in 1917. When he arrived in Loudoun, parents and administrators expressed the expectationthat he would make Loudoun schools competitive with schools in Fairfax, Arlington and other suburbs.

He created a stir almost immediately by altering duties of longtime administrators and sending two central office employees to be principals at remote elementary schools. Parents and students criticized him for tightening classroom disciplinary rules. But he weathered the criticism then.

Recently when the top job opened up in Montgomery County, where he had been an associate superintendent under Harry Pitt, he said he planned to stick with Loudoun schools. "I said when I came to this district that I intended to finish out my career here," Thomas said.