Lulabel Seitz was a few minutes into her valedictory speech at Petaluma High School in Petaluma, California, on June 2 when her microphone went silent.

The incoming Stanford University freshman had been ticking through the challenges that her class had faced — the wildfires that tore through Northern California last fall, a teachers' strike — when Seitz, 17, landed on something personal: what she called the school’s mishandling of sexual assault cases, including her own.

Then the sound went dead.

Seitz later posted the full speech online, claiming that the school administration censored her over “fear of the truth.”

David Stirrat, the principal, didn’t respond to requests for comment from NBC News, but he told a local newspaper, the Press Democrat, that administrators were trying to ensure that the graduation ceremony was “appropriate and beautiful.”

But was it legal for them to silence her?

Lulabel Seitz on graduation day. Courtesy of Lulabel Seitz

Erwin Chemerinsky, a law professor at the University of California at Berkeley and co-author of “Free Speech on Campus,” said four cases have come before the Supreme Court in which public schools punished students’ speech. In three of them, Chemerinsky said, the justices sided with administrators.

But when students in Iowa silently protested the Vietnam War in 1965 by wearing black armbands — and their principal threatened them with suspension — the high court ruled in favor of the teenagers.

Still, none of those cases quite match what happened in Petaluma, Chemerinsky said.

“It’s such an extreme suppression of speech, to turn off the mic,” he said.

But Chemerinsky added that a high school graduation probably would be considered a private forum. “The school can control the message to a large extent,” he said.

School administrators say that’s exactly what they were doing. Assistant Principal Deborah Richardson told the Press Democrat that there were auditions for the graduation speech, and that “the expectation is that the speech you submitted is the speech you will give.”

In an interview, Seitz said she agreed to read that approved speech — but only because she was given no choice.

“They said, ‘Say you’re reading this speech or you won’t get to talk tomorrow,’ so of course I said OK,” she recalled.

That approval process, Seitz added, was new this year.

“It was made just so that I wouldn’t share how the school handles sexual assault cases,” she said.

Matthew Coles, a law professor at the University of California at Hastings, said Seitz’s grievances may add up to a claim that her First Amendment was violated.

“Courts have said for a long time that students don’t shed their First Amendment rights when they go into a public school — unless a student says something that throws the school into disruption or makes it hard for other students to get an education,” he said.

In the “uncensored” version of the speech that Seitz later posted online, she said her classmates persevered against a school "in which some people defend perpetrators of sexual assault and silence their victims."

Coles said this didn’t cross into obscene or other kinds of speech that schools can typically limit “by a long shot.”

“What’s interesting is she did what I think is the most powerful thing she could have done — she posted it on YouTube,” he said. “Probably many more people have heard it now than would have ever heard it before.”