The Fairfax County School Board approved a two-year plan last night for the merger of Jefferson and Annandale high schools that will allow creation of a Northern Virginia science and technology "magnet" school at the Jefferson location.

Under the plan, to go into effect next fall, students who would have been freshman at Jefferson will go to Annandale, while current sophomore and junior classes at Jefferson will be allowed to remain there through graduation, in 1986 and 1987, respectively. Current freshman at Jefferson, the class of 1988, will move to Annandale in September 1987 for their senior year.

"I'm confident that all this planning will work because of the communities involved," said School Board member Laura I. McDowall, who represents the Annandale area.

The merger of the two schools was approved by the School Board in principle in June as one way of addressing the problems of declining enrollments in the eastern part of the county and the increasing importance and popularity of high technology prgorams.

Since then, community representatives and school staff members from Jefferson, located on Braddock Road, and Annandale, about 2 1/2 miles away, have met to determine the phasing-in process. They had recommended the two-year merger that was unanimously approved by the board.

Students favored merging over three years because of difficulties foreseen in transferring extra-curricular activities and awards from one school to the next. They worried, especially, that Jefferson's class of 1987 would work hard for spots on athletic teams and clubs at Jefferson only to be stripped of them in their senior year if they were shifted to Annandale.