Jonathan Zimmerman teaches history at New York University and lives in Narberth. He is the author of Small Wonder: The Little Red Schoolhouse in History and Memory (Yale University Press).

In 1917, shortly after the United States entered World War I, three teachers were fired from a high school in New York City. Their crime was simple: They tried to get students to think about the war....

In times of war, school officials said, teachers needed to rally behind the national cause. The letter to Wilson "could give an opportunity for unpatriotic statements," a New York superintendent explained. "There are some assignments in the world that are not proper for a classroom in a public school, and this is one of them."

And if you think teachers today are free to critique our own wars, you're wrong. As America prepares to commemorate the 10-year anniversary of the Sept. 11, 2001 attacks, indeed, its teachers are arguably less free than at any time since the 1960s. And that should worry anyone who cares about the values that we celebrate on 9/11 itself: human liberty, justice, and freedom....

Consider the fate of Deborah Mayer, a middle-school teacher in Indiana. During a lesson from a student current-events magazine about the war in Iraq, her students asked her if she supported the war; Mayer said she did not. When parents got wind of the discussion and complained, the school district refused to renew Mayer's contract.

And in 2007, a federal circuit court upheld the district's decision. "Expression is a teacher's stock in trade, the commodity she sells to her employer in exchange for a salary," the court said. "A teacher hired to lead a social-studies class can't use it as a platform for a revisionist perspective that Benedict Arnold wasn't really a traitor, when the approved program calls him one."

So if the "approved program" says that the war in Iraq - or in Afghanistan - is a good idea, the teacher can't question that, either....