After the school shooting in Parkland, we are yet again considering arming teachers. We’ve been here before. After Columbine. After Sandy Hook.There’s a certain comfort in the heroic fantasy — "If just one good guy with a gun had been there." But we shouldn’t indulge in fantasy. We should recognize what arming teachers in each of our 98,000 public schools entails.It will cut against over a century of American law. As early as 1870, Texas citizens prohibited taking a firearm into "any school room or other place where persons are assembled for educational … purposes." They weren’t the only ones. Mississippi had a similar law, as did Missouri, Arizona and Oklahoma. The tradition of keeping guns out of schools is so long that even the arch-originalist Justice Antonin Scalia conceded in his District of Columbia vs. Heller opinion that nothing about the Second Amendment disturbs "laws forbidding the carrying of firearms in sensitive places such as schools."It’s going to be expensive. School districts will be liable when a teacher shoots someone wrongfully. All government agents must comply with the U.S. Constitution, including public school employees. As the Supreme Court has insisted, constitutional rights do not "end at the school house door." When teachers search a backpack, when they confiscate a sign, when they expel a student, they are doing these things as agents of the state and the Constitution is in play. In Utah, a drama student accidentally killed himself with a prop gun that shot blanks; his parents sued the teacher for violation of his right to due process. Think how much more often that will happen when the school is full of guns that fire live rounds.It does no good, as some lawmakers suggest, to consider armed teachers simply private persons, "volunteers" who have been "allowed" to carry personal firearms onto school property. The implication is that, should tragedy result, it’s not the school’s fault, it’s the individual instructor’s. But that is a fiction.First, it is inconceivable that the math teacher is a government employee in every interaction at school until the moment she shoots someone dead. Second, who would assume the liability of taking a gun to school without a guarantee the school will indemnify him for any injuries? Third, the Supreme Court has said local governments are responsible for civil rights violations caused by the inadequate training of their agents. Indeed, the quintessential "failure to train" case, according to the justices, is for the government to authorize an officer to carry a gun and never educate him on its proper use.That means there will have to be training, lots of it, and that will be costly. Philip Bump at the Washington Post has crunched the numbers, and his estimates range from the hundreds of millions to the tens of billions of dollars annually. Teachers will have to receive instruction on how to secure their weapons, how to track multiple targets, how to shoot in dim light conditions and how to fire while under stress. They’ll have to learn when to shelter with their students, and when to abandon them and run toward the gunfire; when the boy without a hall pass is reaching for a cellphone, and when he’s reaching for a gun. And they’ll have to learn how to kill today the student they just tutored yesterday.The costs won’t all be monetary. The primary justification for our system of free public education is that a healthy democracy must train each generation to be a citizen. Schools are supposed to equip students with the tools necessary for self-government: deliberation, dialogue, the negotiation of difference, the development of shared goals.As the philosopher Firmin De-Brabander has observed, if we end up turning our public schools into fortresses, if our children come to learn that persuasion is indistinguishable from coercion, and that the guy with the pistol is always right, then we shouldn’t be surprised what kind of society we get as a consequence.I have no idea if arming teachers will make our children safer. But if we’re going to do this, let’s not kid ourselves: A school filled with guards is a prison, and a teacher with a gun is a cop. Darrell A.H. Miller is a professor at Duke University School of Law.