FOR 200 days, India Landry remained seated in protest during the Pledge of Allegiance at her Texas high school. Then, last week, the school's principal informed her that she would have to stand for the pledge or be forced to leave school. She chose the latter.

Ms. Landry, who says she was protesting police violence against black Americans, is now back in class after the school reversed its position following a storm of media attention. But she has filed suit against her principal and the school district, arguing that being forced to stand for the Pledge of Allegiance infringes on her free expression under the First Amendment. She's right. The Supreme Court addressed almost exactly this question in a 1943 decision, holding that students who were Jehovah's Witnesses could not be compelled to salute the flag.

"Freedom to disagree is not limited to things that do not matter much," Justice Robert Jackson wrote then, affirming the American principle that real patriotism requires not enforced conformity but embrace of difference. Unfortunately, Ms. Landry's school is not the only one that would do well to read his opinion — if not as a legal document, then as a moral one. Several Louisiana public schools have announced in recent weeks that student athletes will be required to stand for the national anthem at games. And while private schools aren't constrained by the Supreme Court's ruling, at least two have pulled football players from the field or off the team for anthem-related protests.

Students have always fought for the right to express themselves in schools. What's different now, of course, is that the president has embarked on a multi-week effort to shore up political support from his base by attacking the political expression of NFL players protesting police violence. This weekend, Vice President Pence sought to revitalize the controversy by walking out of a football game after players kneeled during the anthem — an apparent stunt that cost more than $200,000 in taxpayer money to shuttle Mr. Pence to the game and back. Mr. Trump tweeted that he had asked the vice president to leave if players "disrespect[ed] our country."

The players have made clear that their actions aren't intended as an act of disrespect to the United States. As San Francisco 49ers safety Eric Reid wrote, living in "a fair and free society . . . includes the right to speak out in protest." Yet this nuance appears to be lost on Mr. Trump — and, as Ms. Landry's story shows, on others who have taken up the president's misguided call. "This isn't the NFL," a school official reportedly said to Ms. Landry before forcing her to leave.

It's unfortunate that the president has chosen to distort the moral authority of his office into a bully's demand that others adhere to his preferred view of the world. In the absence of leadership from the White House, Jackson's words are more important than ever as a reminder that disagreement and diversity are at the heart of the American experiment.