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The principal of an elementary school in the Virginia suburbs of Washington has apologized for a gym class exercise in which pupils were told to act as runaway slaves for Black History Month.

"The lesson was culturally insensitive to our students and families. I extend my sincerest apology to our students and school community," David Stewart, principal of Madison's Trust Elementary School in Ashburn, Virginia, said in a letter to parents dated Feb. 12 and obtained by NBC Washington.

The incident is especially sensitive for Virginia, whose governor, Ralph Northam, has apologized after it was revealed that a racist photo appeared on his page in his 1984 medical school yearbook, which featured men in blackface and Ku Klux Klan robes. Northam has refused bipartisan calls to resign.

The Loudoun Times-Mirror newspaper, which first reported the Madison's Trust exercise on Thursday, said pupils in third- through fifth-grade physical education classes were given a "game" to play in which groups tackled an obstacle course meant to represent the Underground Railroad, the network of secret routes that slaves used to escape to free states and Canada during the 19th century.

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At a school board meeting last week, Michelle Thomas, president of the Loudoun County chapter of the NAACP, told the board that she learned of the exercise last month in a letter that said pupils were given the choice of being a slave, a sharecropper or a landowner.

But "slavery was not a joke," she said. "You didn't get to choose."

Thomas said children "don't need to relive slavery. We've done enough. We've paid enough."

She reiterated, "You had kids playing runaway slaves," telling the school board: "You need to run away."

Linda Deans, a retired educator, told the board that "having a black male student crawling around on the floor in the dark as he re-enacted a lesson about a runaway slave is an offensive educational strategy."

"Corrective action is required now, not next school year," Deans said.

Stewart, the principal, said in his letter to parents that "this incident has revealed a need for us to further explore how we can ensure this will never happen again."

He said the material taught in the exercise would be presented in a different way "to ensure that all the students have a full understanding of the material, within an appropriate and respectful context."

He also said he would appoint an "equity/culturally responsive team," details of which he didn't provide.

Thomas told NBC Washington on Thursday that she was "satisfied with the progress that we're making."