The Fairfax County School Board unanimously agreed last night to increase the freshman class at Thomas Jefferson High School for Science and Technology by 50 students, saying it is unfair that the size of the elite magnet school has remained virtually unchanged despite the county's growth.

The new policy, which will increase the size of each incoming class from about 430 to 480, will begin with freshmen selected for the Class of 2009.

School Board member Ilryong Moon (At Large), who proposed the change, said that although the student population in Fairfax has grown dramatically over the years -- from about 128,000 in 1990 to 166,000 this year -- the number of students accepted into the highly selective school each year does not reflect that growth.

Moon also noted that the magnet school accepts students from other school districts across Northern Virginia that are growing rapidly, including Loudoun, Prince William and Fauquier counties.

"We have many qualified students out there, and because of limited space, we've had to turn them away," Moon said. "We have a chance to provide some of those students the opportunities that TJ offers."

School officials said the change will mean an initial outlay of about $390,000 to outfit the school, including about $250,000 for two more science labs and $140,000 for trailers to accommodate the new students and those displaced from classrooms converted to labs. The cost of adding teachers and other staff was not immediately available.

In recent years, school officials have been debating admissions policies at Thomas Jefferson because of concerns that few Hispanic and black students attend the school.

Earlier this month, the School Board approved a new admissions process in an effort to create a more diverse student body. The new policy directs admissions officers to give more consideration to grades in making the first cut, instead of relying largely on a standardized test. Then, as they decide who will make the second and final cut, admissions officers read essays and teacher recommendations and consider factors including race, ethnicity, poverty and cultural experiences.

The new admissions policy also stresses that TJ students should have an aptitude for and interest in math, science or technology, a reflection of concerns that its curriculum has become more generalized.

Thomas Jefferson, which opened as a magnet school in 1985, generally admitted about 400 freshman each year. In 2001, in an effort to increase diversity, the School Board voted to increase the freshman class by 30 qualified students from underrepresented neighborhoods.

Jeanine Martin, a member of the county's Gifted and Talented Advisory Committee and the parent of a TJ graduate, said she does not object to the addition of 50 students each year but said she thinks the board made the change in an effort to placate people who opposed the new admissions policy. Martin and others worry that the change could lower the school's standards.

"We still have the same concerns we had about the change in admissions," Martin said. "I think they think that adding the 50 will appease people."

But board member Jane K. Strauss (Dranesville) said the quality of courses will not suffer. "We are committed to the course offerings and the syllabus and the class size at Jefferson," she said, "and that we will not change."