Almost a week after President Donald Trump demanded that NFL players who protest the national anthem be fired, one high school principal threatened to punish student-athletes who decided to sit or kneel in protest during in-game anthems.

Waylon Bates, the principal of Parkway High School in Bossier City, Louisiana, issued a letter to parents and students Thursday that highlighted the school’s policy regarding expected behavior during the national anthem. Players and coaches must stand during the anthem out of respect, the letter stated.

"The LHSAA (Louisiana High School Athletic Association) allows school principals to make decisions regarding student participation in the National Anthem while competing in athletic contests and games,” the letter read.

“Parkway High School requires student athletes to stand in a respectful manner throughout the National Anthem during any sporting event in which their team is participating," it continued. "Failure to comply will result in loss of playing time and/or participation as directed by the head coach and principal. Continued failure to comply will result in removal from the team. Parkway High School is continued to creating a positive environment for sporting events that is free of disruption to the athletic contest or game."

The letter arrived after multiple NFL players and team owners locked arms and kneeled in symbolic protest during Sunday’s Week 3 games. 

The players' protests came in response to comments made by Trump, who during a Friday rally in Alabama suggested that NFL players like former San Francisco 49ers quarterback Colin Kaepernick — who first kneeled in protest last year — be fired from their positions.

North Carolina Tar Heels backup quarterback and Parkway High School graduate Brandon Harris took to social media Thursday in response to the school’s letter.

"Man, this is my high school. Never thought this would hit so close to home," he wrote on Twitter.

The letter also prompted a reply from the American Civil Liberties Union of Louisiana (ACLU), which warned local schools that "forcing students to stand during the national anthem or punishing students who ‘take a knee’ in protest of racial injustice and police brutality would violate students’ First Amendment rights."

"Bossier Parish is threatening to punish students for peacefully protesting racial injustice and taking a principled [stand] for freedom and equality," ACLU of Louisiana executive director Marjorie Esman told the Washington Post. "This is antithetical to our values as Americans and a threat to students’ constitutional rights."

Supreme Court rulings protected students and could not force them to participate in patriotic ceremonies, Esman said, according to the Washington Post.

"Schools have no valid interest in turning their students into mouthpieces of government speech," she said. "Indeed, schools should respect students who embrace their constitutional rights and stand up to injustice — not punish them. And it would be patently unconstitutional for the school to do so."

The ACLU also took to Twitter and posted a message regarding the rules.

"The Supreme Court ruled that students don't have to stand for the Pledge of Allegiance — that goes for the national anthem on the ballfield too," the ACLU tweeted.