[Jonathan Zimmerman teaches history and education at New York University.]

Can a high school student display a banner that says "Bong Hits 4 Jesus"? That's the question the Supreme Court debated this week in a case pitting the free-speech rights of the student against the duty of his school to warn against the dangers of drugs.

But in this case, both sides are wrong. The banner was a foolish vulgarity, unworthy of court-affirmed protection for politically meaningful speech. Under the school's theory, meanwhile, educators could censor any speech that contradicted their goals or ideas. And that might be the scariest idea of all.

This strange saga began five years ago in Juneau, Alaska. The school let students leave the building to see the Olympic torch en route to the 2002 Winter Games. As the torch passed by, senior Joseph Frederick unfurled his banner. Principal Deborah Morse ordered Mr. Frederick to take it down; when he refused, she tore it down. Later, she suspended Frederick for 10 days. Frederick sued, citing the Supreme Court's 1969 warning – in Tinker v. Des Moines – that students do not "shed their constitutional rights to freedom of speech or expression at the schoolhouse gate."

Technically, Frederick was outside the "schoolhouse gate." But no matter where he was, Frederick insists, he had the right to display his banner.

That's where he's wrong. In the Tinker case, a school tried to stop students from wearing black armbands to protest the Vietnam War. Ruling in favor of the students, the Supreme Court took pains to distinguish their "pure speech" – that is, speech of a political nature – from other forms.

Only political speech merited protection, the court said, noting the special purpose of education in a democracy: to develop free and independent minds. Public schools "may not be enclaves of totalitarianism," the court admonished. Instead, schools should promote "a robust exchange of ideas."

So what "exchange of ideas" did Frederick aim to provoke?

None. As Frederick later testified, he chose his slogan "to be meaningless and funny" and "to get on television." He recently admitted that the phrase was irrelevant. "I wasn't trying to say anything about religion. I wasn't trying to say anything about drugs," Frederick said. "I was just trying to say something."

But students do not have the right to "say something" – or anything – just because they feel like it. That's why they're not allowed to shout obscenities in class. Such behavior doesn't add anything to a school's "robust exchange of ideas."

But under a theory put forth by the White House, which sides with Ms. Morse, the principal could have censored Frederick even if his speech did promote intellectual exchange. Under the 1994 Safe and Drug Free Schools and Communities Act, the Bush administration argues, schools receiving federal money must "convey a clear and consistent message" that illegal drug use is "wrong and harmful." So Fredericks's banner would have to go. But so would any student critique of drug policies.

Drugs are particularly controversial in Alaska, where adults may legally possess a small amount of marijuana. Under the White House theory, though, even a banner that said "Legalize Pot" – a clear political statement – might be censored.

Schools might even be allowed to muzzle student criticism of "No Child Left Behind" (NCLB), the Bush administration's signature education reform. In a brief written by Kenneth Starr, (yes, that Kenneth Starr), Morse argued that officials may censor any student speech that interferes with their "educational mission." For good or ill, NCLB now lies at the heart of every public school's mission. Under Mr. Starr's theory, then, students might be barred from speaking ill of it.

Yet both sides of this dispute have lost sight of schools' true educational purpose: to foster critical and independent thought....