Gary L. Jones, named to the No. 3 job in the department of Education, said yesterday he plans to remain on the Fairfax County School Board unless federal lawyers say it would constitute a conflict of interest.

Jones, 36, a member of the school board for almost four years, said Education Secretary Terrel H. Bell has encouraged him to retain his $5,500-a-year county post. But Jones said government lawyers and an ethics panel will have to decide whether he can continue on the board and accept his appointment as deputy undersecretary of Education for policy and budget planning.

The federal job would place Jones in charge of fiscal planning for a department President Reagan has said he would like to abolish. Even though Jones said he expects the president to "either reorganize or dismantle" the agency, he said he had "no reservations whatsoever" about accepting the $50,000-a-year presidential appointment. "To be a part of assessing what role the federal government should be playing in education is a very exciting challenge at this time," he said.

As a member of the Fairfax School Board, one of Jones' current problems is assessing what the county should do if the Reagan administration is successful in eliminating millions of dollars in federal aid that Fairfax receives. Jones said yesterday he favors less federal regulation of education and will support a Fairfax move to recoup an estimated $9 million loss in impact aid by charging tuition to the families of school children who live on military bases in the county.

Jones, once executive assistant to former Sen. Robert Giffin (R-Mich.), was appointed to the Fairfax board under the sponsorship of County Board Chairman John F. Herrity, a Republican. Jones' term will expire in July and he said it will be up to Herrity, and the federal lawyers, to determine whether he can continue on the board after then.

Jones, who worked in Reagan's presidential campaign and later on his transition team, has been grants director for the the MacArthur Foundation of Chicago.