Half of the 12 Fairfax County School Board members have chosen not to seek reelection, guaranteeing that the board that oversees the region’s largest school system will undergo its most substantial turnover since 2003.

The school board race took firm shape Tuesday evening as the deadline for declaring candidacy in the November elections came and went.

Four board incumbents face challengers. That raises the possibility of the biggest change in the Fairfax board since it became an elected body 16 years ago.

“I think what this election comes down to is the community having a chance to come forward and say, ‘We are looking forward to fresh leadership,’ ” said Megan McLaughlin, a parent activist seeking the Braddock District seat being vacated by incumbent Tessie Wilson. Nell Hurley, a parent and longtime schools volunteer, is also a candidate for the seat.

McLaughlin co-founded Fairgrade, an organization that led a successful campaign to ease a stringent county grading policy that some argued hurt students’ chances of getting into competitive colleges. She and some other candidates expressed frustration with the board and Superintendent Jack D. Dale.

Board Chairman Jane K. Strauss (Dranesville), a longtime member who is seeking reelection, said complaints that the board doesn’t listen to parents and teachers are unfounded. She pointed to Fairfax’s student achievement — which has never been higher, according to state test results — and narrowing achievement gaps as proof that the 175,000-student school system is doing its job.

The race for three at-large seats is crowded. Of eight declared candidates, only Ilryong Moon is an incumbent.

The others are:

• Sheree Brown-Kaplan, a special education advocate;

• Steve Stuban, who became a critic of the system after his son Nick committed suicide amid the fallout of a disciplinary infraction;

• Lin-Dai Kendall, who advocates protecting taxpayer dollars and whose children have attended Robinson Secondary School;

• Lolita Mancheno-Smoak, a native of Ecuador who says she wants to be a voice for minority communities;

• Ryan McElveen, a 25-year-old Vienna native and former class president at Marshall High School;

• Ted Velkoff, a former PTA president at Chantilly High School;

• Catherine Clark, an educator who founded a preschool in California before moving to Fairfax in 2007.

Five incumbents are retiring in addition to Wilson: Brad Center (Lee), Stuart D. Gibson (Hunter Mill), James L. Raney (At-Large), Martina A. Hone (At-Large) and Elizabeth T. Bradsher (Springfield).

Four are seeking reelection in addition to Moon and Strauss: Daniel G. Storck (Mount Vernon), Sandy Evans (Mason), Patty Reed (Providence) and Kathy L. Smith (Sully). Evans and Reed are running unopposed.

Other candidates are Pat M. Hynes and Nancy A. Linton in the Hunter Mill district; Tamara J. Derenak Kaufax, running unopposed in Lee; Michele Nellenbach in Mount Vernon; Sheila P. Ratnam in Sully; Louise Epstein in Dranesville; and Elizabeth L. Schultz and John F. Wittman in Springfield.

Storck, a board member since 2004, said he has long sought to improve the board’s public communication. Nellenbach, a PTA activist who wants to unseat him, said: “I just feel we need more of a vocal advocate who asks more questions and pushes more.”