The Fairfax County Board of Supervisors yesterday appointed Joy Korologos, a part-time school secretary, to fill the unexpired term of school board member Gary L. Jones, who recently became the number three man in the Department of Education.

Korologos, 44, was nominated for the at-large school board seat by supervisors chairman John F. Herrity. Korologos said she worked for Herrity in his first supervisor campaign in l971. She also was appointed by Herrity to serve on the search committee that recommended Jones for the school board position more than four years ago.

The newly appointed school board member said she resigned Monday from her part-time secretarial position at Forestville Elementary School in Great Falls to devote full time to the $5,500 a year job on the school board. Korologos has worked part-time for the county school system for five years.

"I have no crusades to carry to the board," said Korologos, who calls herself a political conservative. "But I feel I can bring some nuts and bolts ideas to the board."

She replaces one of the most conservative members of the school board. Jones gave up the seat in early October, three months after he was appointed to his third two-year term, under pressure from Reagan administration officials urging its appointees to avoid conflicts of interest.

Jones was selected as deputy undersecretary of Education last April by the Reagan administration, but said he delayed resigning from the school board until his Senate confirmation in early October because administration offices were divided over whether his dual roles violated federal ethics rules.

Herrity said he reappointed Jones to the school board despite warnings that Jones may be forced to step down, "because we had a 50-50 shot of keeping him on the board."