The Arlington School Board should spend more money to improve science and mathematics programs because it decided not to send students to Fairfax County's new regional high school for science and technology, several members of school and community groups said last night.

The board decided last fall not to participate during the 1985-86 school year in the Thomas Jefferson High School for Science and Technology, one of four state-designated "magnet" schools.

Board members have proposed spending $39,000 to improve science programs but several speakers at last night's public hearing on the budget said that was not enough.

Conchita Mitchell, president of the Arlington County Council of PTAs, urged additional funding for math and science classes, saying the council would lobby the County Board for more school funding for these and other improvements.

In the last scheduled public hearing before a budget request goes to the County Board Feb. 28, School Board members added a new twist to the yearly round of debate over funds by putting their thoughts about the budget on paper.

The board circulated a list of accounts and the amounts they proposed to add or substract from them, then opened the floor to comments on those changes.

The board based its suggestions on the $71.9 million "tentative tier," the lower of two budgets Superintendent Charles E. Nunley presented last month. Nunley described the lower budget as a "bare bones" proposal and supported a higher figure, a $74 million "management" budget.

"We decided to use the tentative tier as a base to work from, because we really wanted to be in the mode of adding rather than subtracting," said Board Chairman Gail H. Nuckols. She said the board prepared the list of accounts to involve the community more thoroughly in the budget process.

The largest addition in the board's list is $300,000 to implement an optional seven-period day and expand Saturday classes. The board voted the plan at its last meeting in response to complaints that students did not have room in their schedules for fine arts, music and other elective classes.

Under the plan, students choosing an additional elective would take it in place of one of their required courses. They would then take the required course in three 85-minute periods following school hours.

This plan was criticized last night by Jerry Mayer, president of the Washington-Lee High School class of 1986, who said, "After talking to numerous students, I can with confidence say that this extra period proposal is not popular." He claimed the long after-school periods to take regular courses would prevent students from participating in sports, plays or other extracurricular activities.