Nearly 20 Lansdowne residents addressed the Loudoun County Board of Supervisors at a public hearing Tuesday, asking that county leaders reconsider their decision to build a high school, known as HS-8, at a controversial site neighboring the National Conference Center.

Despite the latest round of opposition, the board took no action to address the pending contract, effectively ensuring that the deal would go forward after the expiration of a due-diligence review deadline Friday. The board voted in January to extend the due-diligence period, giving county officials an additional month to complete a review of the $20 million contract to buy 45.6 acres from the National Conference Center in Lansdowne.

The previous Board of Supervisors unanimously approved the contract in September, following lengthy debate among residents of the neighborhood surrounding the site.

Although county leaders have maintained that the proposed site offers the best location at the best value, residents have questioned the financial reasoning behind the purchase. The $20 million cost is vastly higher than the per-acre value of the entire 112-acre parcel, which was assessed at about $22.5 million in July, according to county records. There are also two liens on the property, records show.

Some community residents welcomed the new school and said they were pleased that their children would be within walking distance of the campus. Many said that the school, which county leaders have referred to as the final component of a “three-school solution” for easing crowding in the Ashburn area, was desperately needed as the populations of nearby high schools continue to rise.

But other residents have voiced growing concerns, many of which were repeated at Tuesday’s hearing. They have pointed to the high school increasing traffic on generally quiet neighborhood roads and say they worry about how traffic backups will affect their morning commutes. Others have questioned the environmental consequences of building a school in an area that includes wetlands, a flood plain and a stream that feeds the Potomac River.

Residents have also protested the potential loss of a proffered community park — including public sports fields supported by investments from nonprofit youth athletic organizations — that would be become part of the new school’s property.

John Powers, a Lansdowne resident and outspoken opponent of the site for HS-8, voiced his concerns about the high school’s proximity to a nearby sports bar and to the conference center, which hosts events that serve alcohol. He questioned whether the location might pose a conflict with the school as a drug- and alcohol-free zone.

Powers said more than 200 households in Lansdowne remain firmly opposed to the placement of the high school in their neighborhood. But Supervisor Ralph Buona (R-Ashburn), who represents the community, has said that a recent poll found that the vast majority of the residents in the area would welcome the school.

Powers said that he understands the need for a new high school to relieve crowding but that the National Conference Center land is not the right place.

“We’re hoping that the board understands that we know we need another high school in the Ashburn area, but this is not the site,” he said.