Gary L. Jones, picked by the Reagan administration for the No. 3 job in the Department of Education, remained on the Fairfax County school board for six months despite warnings from federal officials that he should step down to avoid a conflict of interest.

On Thursday, two days after the Senate finally confirmed him to be deputy under secretary of the department, Jones disclosed that he had given up his fight to remain on the school board.

During that period between his nomination and confirmation, Jones won renomination to the school board, saying that administration officials were divided over whether his dual roles violated federal ethics rules.

After his resignation was disclosed, Jones, a four-year member of the Fairfax school board, said that White House lawyers had advised from the outset that he should leave the board. But Secretary of Education Terrel H. Bell and several members of Congress were urging him to remain in both positions, Jones said.

Jones, 36, a Reagan campaign worker and once executive assistant to then-senator Robert Griffin (R-Mich.), said he had delayed his resignation because "I was hoping against hope that I could convince the powers that be that there was no cause for concern. It didn't work out.

"The general counsel for Reagan thought there might be the appearance of a conflict of interest," said Jones. "They preferred that I not try to hold a dual role.

"It was a close call," said Jones. "But when you're a presidential appointee and the president's attorneys say, 'We'd rather you didn't remain on the board,' you say okay."

Jones said he realized in late August, barely two months after his appointment to a third term on the school board, that he would be forced to step down. He did not write his letter of resignation until Sept. 30 and it was not released until Thursday night.

The letter, to John F. Herrity, chairman of the county supervisors, made no reference to White House suggestions that Jones give up the job; instead it blamed the decision on "the considerable time restraints inherent in my position at the U.S. Department of Education.

"I have concluded that it is impossible for me to give appropriate atttention to my responsibility as a board member while at the same time fulfilling my responsibility to this administration," the letter said.

During the past months, Jones, who held an at-large position on the board, has excused himself from voting on several issues that involved the federal government. As a member of the 11-member Fairfax school board, Jones was paid $5,500 a year.

If Jones is successful in his $50,000-a-year position at the department, he could be out of a job in less than a year. Jones is in charge of carrying out Reagan's proposal to dismantle the department, which was created during the Carter administration.

Jones has jumped in and out of Reagan jobs three times in slightly more than two years. He began during the 1980 campaign, working for Reagan campaign manager John Sears. When Reagan fired Sears, Jones also lost his job.

Former Treasury secretary William Simon then hired him as vice president of the private MacArthur Foundation in Chicago. Jones took a leave from the job and returned to the Reagan fold after the election as a member of a transition team.

Because school board appointments in Virginia are considered to be nonpartisan positions, Fairfax officials said they saw no problem under state laws in Jones holding both offices.