When students walked out of Garnet Valley High School in Glen Mills last week, they demonstrated the value of free speech. But when sophomore Clayton Bromley refused to participate, his classmates made it clear that his own speech wasn't valued at all.

"They started yelling at me," Bromley told Philadelphia Magazine. "They singled me out. They made me look like a bad person."

It's easy to celebrate freedom of speech when it's speech that you like, of course. But if we fail to protect speakers whose ideas we don't share, free speech becomes a dead letter. And that leaves us all less informed, less aware, and less alive to the world around us.

On the same day that students walked out of their high school in Rocklin, Calif., history teacher Julianne Benzel was placed on administrative leave. Her misdeed? Raising questions about, yes, the walkout itself. During the days preceding the protest, Benzel asked her class whether students demonstrating against abortion would be allowed to leave school as well.

It's a great question. Does her school support student protest, for any reason? Or are some reasons more valid than others? But that's not a debate the students are likely to encounter anytime soon. The school district said it had received "several complaints from parents and students" about what Benzel said in class. Now other teachers will probably shy away from the issue, lest they provoke similar objections.

Meanwhile, here in Philadelphia, Penn Law professor Amy Wax faced a torrent of hostility for comments she made about blacks' academic performance. "I don't think I've ever seen a black student graduate in the top quarter of the [Penn Law School] class, rarely in the top half," Wax told Brown University scholar Glenn Loury, in a call-in show recorded last fall that recently came to light. "I can think of one or two students who've graduated in the top half of my required first-year course."

Wax had already sparked outrage last summer, with an Inquirer column attributing poverty and other social ills to Americans' abandonment of midcentury "bourgeois culture." But her attack on affirmative action — which, she alleges, leads to the low performance of underqualified minorities — was apparently one step too far. Facing demands for Wax's dismissal, law dean Ted Ruger announced that she would be removed from the required class she instructs.

"In light of Professor Wax's statements, black students assigned to her class … may reasonably wonder whether their professor has already come to a conclusion about their presence, performance, and potential for success in law school and thereafter," Ruger wrote.

But in light of last week's events, students and faculty of every color — and every political perspective — may reasonably wonder whether it's safe to discuss race and affirmative action at all. And I suspect many of them will conclude that it's not.

Indeed, on this issue, professors have been muzzling themselves for a long time. In a 2006 survey, 43 percent of American faculty opposed race-based affirmative action in college admissions. But you rarely hear from them, because — with few exceptions — they bite their tongues. That's not good for our universities, or even for affirmative action, which could only benefit from a full and open debate.

Dean Ruger said all the right things about free speech, emphasizing that Amy Wax and everyone else "will remain free to express their views." He also provided important corrections to Wax's comments, noting that black students "have graduated in the top of the class at Penn Law."

But Penn officials didn't say anything about the overall level of black academic achievement at the law school, and I doubt they will. If there's a racial achievement gap at Penn Law, as there is in other parts of our educational system, we should know about it — and talk about it — so we can take action to close it. But that, too, has become less likely now.

Too controversial. Too dangerous. Too hot to handle.

And that's bad news for all of us. Like it or not, affirmative action is one of the most hotly contested issues in American law and society. How will people learn about the issue — in all of its complexity — if they hear only one side of it?

That's a particularly strong danger in our hyper-polarized political environment, where increasing numbers of Americans report that they never talk with people from a different party or perspective. And here's the most depressing fact of all: The more education you have, the less likely you are to discuss politics with someone across the political aisle.

That's the key takeaway from Bill Bishop's still-useful 2008 book, The Big Sort. In theory, education should force us out of our ideological cocoons and comfort zones. But in practice, it more often reinforces our prejudices and predilections.

On affirmative action, I'll admit, my ideas are different from Amy Wax's. And I understand why some people were insulted by her remarks. But I also believe our intellectual lives will be enhanced by hearing what she has to say, no matter how hard it might be to listen.

And there's only one cure for that: a loud and boisterous culture of open dialogue and exchange. If we restrict or censor speech, we won't learn how to analyze different ideas. And if we're constantly looking over our shoulders, wondering if we said the wrong thing, we won't learn how to speak across our differences.

It's always tempting — and, let's face it, lots of fun — to stamp out the pariah in our midst. But when censorship wins, everyone loses. That's a lesson we all need to learn, over and over again, until we know it by heart.

Jonathan Zimmerman teaches education and history at the University of Pennsylvania. He is the author (with Emily Robertson) of "The Case for Contention: Teaching Controversial Issues in American Schools" (University of Chicago Press).