The Fairfax County School Board has decided to consider more than grades and test scores in selecting students who will attend a new regional high school for science and technology in Annandale.

Responding to pressure from citizen groups, the board voted 6 to 3 early yesterday to base the initial screening of 800 applicants on grade point averages and test results. But final selection of the school's 400 freshmen will be the result of interviews, questionnaires and recommendations, the board decided.

Some organizations had criticized how the school staff planned to fill the 400 positions in the freshman class, saying a selection based on grades and test scores might not attract the "basement computer hacker" and other creative students.

The school board's decision came several hours after a unanimous vote to open the new high school to students from seven Northern Virginia jurisdictions.

The vote means that the school, originally planned for Fairfax County students, will be a state-designated "magnet" institution also serving Arlington, Loudoun and Prince William counties and the cities of Alexandria, Falls Church, Manassas and Manassas Park.

The school board, however, did not extend the arrangement beyond the school's first academic year, which will begin next fall. For it to continue beyond then, contracts with the other seven school districts will have to renewed before the fall of 1986.

The school, to be called the Thomas Jefferson High School for Science and Technology, will be in what is now Jefferson High School, located off Little River Turnpike in Annandale. In its first year, it will have 400 ninth graders and a special one-year program for 200 seniors.

As one of four regional magnet schools chosen by Virginia Gov. Charles S. Robb, the school will receive $242,000 in state funding for planning and start-up costs in 1985 and 1986.

Among the other decisions made by the board in the meeeting that ended yesterday morning:

Graduation requirements for the school's first freshman class will be set at 25 credits, three more than for Virginia's advanced studies diploma. Students will have to complete three years of one foreign language, mathematics through calculus, and a four-year science sequence.

Faculty at the school would receive a 7 percent salary increase because the workday at the school would be a half hour longer than the normal 7 1/2 hours.

The school day would be divided into nine 45-minute periods, including lunch and an extracurricular activity.

School board members had expressed concern that, as a regional school, it would not be under Fairfax control. But under the agreement approved yesterday, the other school districts only would play on an advisory role in administering the school.

Fairfax School Superintendent William J. Burkholder made clear that the agreement is not negotiable by the other jurisdictions. "If you don't like it, don't sign it," is how he described the Fairfax's position. He said the other seven superintendents "clearly" understand that provision and that one or two jurisdictions would not go along with the pact.

School Board Chairman Mary Collier and member Laura McDowall said the agreement should guarantee that students could graduate from the high-tech school if their district pulled out at the end of an academic year.

One worry the Fairfax Board has had is that the presence of students from other jurisdictions would keep some Fairfax students out of the school. The regional agreement requires that the total number of students accepted from any district not exceed the percentage of that district's school population in Northern Virginia.

Under current enrollment figures, that would mean Fairfax students would make up at least 64 percent of the regional school's student body. Burkholder said Prince William students would make up 16 percent; Arlington, 7 percent; Loundoun, 6 percent; Alexandria, 5 percent; Manassas, 1 percent; Manassas Park, .5 percent; and Falls Church, .5 percent.

The proposed agreement with the school districts requires that they reimburse Fairfax for the cost of educating their students. David Sawyer, who is coordinating the preparations for the school, estimates tuition will cost $4,098 per student.