President Donald Trump on Thursday suggested school security should be akin to that of banks.

Trump has proposed arming teachers and other professionals at schools as a way to prevent mass shootings.

That's not how bank security works.

At the White House on Thursday, President Donald Trump said, "I want my schools protected just like I want my banks protected."

He said this as part of his proposal to arm large numbers of teachers — as many as 40% of them, he thinks, may be qualified.

Well, how are banks protected? Not by arming the bank tellers.

While Trump has conjured an image of armed teachers using their guns to heroically stop a shooter, that's not how banks work at all.

When I worked at Wells Fargo, I had to go through the online bank-branch security training like all other employees. The instruction in the event of a robbery is to hand over the cash in your cash drawer to the robber and hit the silent alarm if it's safe to do so.

The alarm summons the police: professionals with guns. And that's all for good reason. A bank teller's job is to conduct bank transactions, not to engage in armed defense of the bank. They don't have the right skills for armed defense of the bank.

Some major bank branches have armed security, but most don't. If they do, it's people whose specific job is to provide security.

One reason you don't need armed personnel in banks is there are passive security measures — bank tellers may be behind protective glass, most of the money is inaccessible in time-locked safes, etc. Few of these measures are transferable to schools; we can't put bulletproof glass between students or lock them in time-controlled safes.

Trump seems to recognize this, because when he talks about the need to "harden" schools, he doesn't talk about defensive measures like bulletproof glass, but about offensive measures — that is, a lot of staff members with guns who can shoot back.

But the main reason you don't need armed personnel in banks is that policing and law enforcement provide the most effective deterrent to bank robbery.

So banks may have some security lessons for schools. But they're not the lessons Trump imagines.