The Fairfax County School Board voted last night to open Centreville's new Braddock Park school next year with students from grades seven through 10, but to allow area 10th graders to stay in their current high schools if they wish.

The board's 7-to-2 decision came after many parents objected to school administrators' original proposal to open the new school with ninth and 10th graders, and a limited number of seventh graders.

The Braddock Park school, which does not yet have an official name, will include students from southern and western parts of the county now served by Fairfax and Chantilly high schools.

Parents from those two schools disagreed over which grades the new schools should include. Many parents from the Chantilly objected to including younger students, fearing they would encounter social and academic problems if mixed with older students.

Board members Letty Fleetwood and Laura McDowall, who voted against the plan, objected that making attendance for 10th graders optional might hurt the school's program for ninth graders.

Many Fairfax parents had urged a plan like the one adopted, fearing that a new school with high school students only would drain too many students from their school. At a public hearing Tuesday on the issue, "the recommendation at that time was not overwhelmingly accepted," School Superintendent Robert R. Spillane said last night. "I believe there was only one {person} who spoke in favor of it."

The board's final decision, similar to one Spillane suggested last night, would relieve overcrowding at Chantilly, and still provide a good program for younger students. By making the 10th grade attendance optional, students who are already enrolled at another hgh school would not have to leave their present classes if they do not want to, he said.

The board vote gives Spillane the option of opening the school without a 10th grade if fewer than 250 local 10th graders sign up to attend.

The new school will be phased in as follows: grades seven through 10 in September 1988; seven through 11 in 1989-90; seven through 12 in 1990-91, and nine through 12 in 1991-92, when a new intermediate school is to open.

In other business, the board is to vote Dec. 3 on a proposal to spend $821,700 in additional revenue as part of its annual midyear budget review. The money comes from savings in the fuel budget and additional federal and state aid.

Under the proposal presented by Spillane's staff, half the money -- $461,998 -- would go for vehicle maintenance and operation. Increases in salaries and overtime for some school employes would account for most of the remainder.