The Fairfax County School Board approved $1.5 million in additional spending for the current school year last night, including a new program for troubled seventh and eighth graders.

The new program for "socially maladjusted youth," to be housed at the former Bryant Intermediate School in the Groveton area, was modeled on a school in Frederick County, Md.

The pilot program, approved on a unanimous vote, will enroll 25 students and have a staff of nine when it gets under way in January. It will cost $195,000 for the first semester's operations. The program will be evaluated at the end of the 1987 school year, when the board will decide whether to continue it.

Other major budget expenses include $129,000 to add a third deputy superintendent's job, for financial services, and an assistant and secretary; $112,850 for textbooks and supplies at the new Thomas Jefferson High School for Science and Technology in Annandale, and $365,000 to improve science laboratories at West Potomac High School, which houses the merged Fort Hunt and Groveton high schools in the former Groveton building.

Also, $399,170 for classroom furniture and equipment, including that needed to accommodate enrollment growth next year, and $342,250 for electric typewriters and associated wiring to replace manual typewriters used in business classes.

The money for the new spending, to be added to the board's half-billion-dollar budget, comes from additional state and federal aid and other outside revenue. Additional county revenue will not be required.

In other business, a high school senior urged the board to eliminate "cruel and senseless" dissection of preserved animal specimens in biology classes and substitute films, models, diagrams or computer programs.

Pam Sterling, 17, a student at South Lakes High School in Reston, said she and other members of a group called Students for Animal Protection have gathered 900 student signatures against such dissection in the last six weeks.

School officials said county policy discourages unnecessary dissection and allows students to perform alternative activities if they object to dissection on philosophical grounds.