The Virginia State Board Education, rejecting the advice of its staff, established yesterday a passing score of 70 percent of competency tests in reading and mathematics that students throughout the state will have pass in order to graduate from high school.

The first set of exams - 60 multiple choice questions in reading and 99 questions in math - will be given next week to all of the state's 90,000 high school sophomores.

Those who do not pass now will be given several chances to take the test again. But under a law, adopted by the state legislature last March, all students will have to pass it - in addition to their regular courses - before they can graduate from a high school in Virginia starting in 1981.

Last month the state board's staff suggested that no passing score be established until the results of the first round of testing were in.

Board Chairman Henry W. Tulloch said the board rejected this advice because it wanted set the passing score based on a judgment of the minimum skills high school graduates should have. Tulloch said the board wanted to avoid charges that the standard was fixed for political reasons to avoid too many failures.

At yesterday's meeting in Richmond the nine board members voted unanimously to set the passing grade at 70 in both parts of the exam.

Assistant Superintendent Richard L. Boyer said all nine board members took the test themselves during the past month. It has also been reviewed by a panel of 91 educators, parents, and community members from throughout the state. Boyer said the laymen generally wanted a higher passing grade than the educators.

The state board also decided to endorse a proposal to require all new teachers to pass a written competency exam before they are licensed. It said it would help a joint legislative committee find a suitable test, which requires approval by the legislative before it can be adopted.