San Francisco 49ers safety Eric Reid (35) and quarterback Colin Kaepernick (7) kneel during the national anthem before an National Football League game against the Los Angeles Rams in Santa Clara, Calif. The movement began more than a year ago with a Kaepernick protesting police brutality against minorities by kneeling silently during the national anthem before games. (Marcio Jose Sanchez/AP)

A Louisiana public high school principal who ordered athletes to stand during the national anthem or face penalties is coming under fire from civil rights organizations that say the school is violating students' First Amendment rights.

In a letter sent to student athletes and parents Thursday, Waylon Bates, the principal of Parkway High School in Bossier City, La., said the school "requires student athletes to stand in a respectful manner throughout the National Anthem during any sporting event in which their team is participating."

Bates warned that "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 committed to creating a positive environment for sporting events that is free of disruption to the athletic contest or game."

[Trump demands NFL ‘fire or suspend’ players or risk fan boycott]

Bates's letter comes in the wake of symbolic protests by professional athletes last weekend. More than 200 NFL players took a knee or sat during the national anthem at games to protest racial injustice and in response to comments by President Trump, who likened those who protest to a "son of a b----" and said they should be fired.

On Monday, nine members of a girls soccer team in Maine knelt during the national anthem, and high schools around the country are anticipating that some of their athletes will protest in similar fashion this weekend.

The letter from Parkway High School, which is in Bossier Parish in northwestern Louisiana, drew an immediate response from the American Civil Liberties Union of Louisiana, which put Louisiana schools on notice and warned them 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," said ACLU of Louisiana executive director Marjorie Esman. "This is antithetical to our values as Americans and a threat to students' constitutional rights."

Esman pointed to Supreme Court rulings that found state schools cannot force students to take part in patriotic rituals.

"Schools have no valid interest in turning their students into mouthpieces of government speech," she said in the statement. "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."

Parkway's principal did not immediately return a call for comment Thursday.

Parkway High's next football game is at 7 p.m. Friday.