“When a man knows he is to be hanged in a fortnight, it concentrates his mind wonderfully,” Dr. Johnson once quipped.

This can happen even when the person is not quite a man, or woman, but a teenager. A high school student, say, and the threat isn’t the certainty of being strung up in two weeks but the possibility of being gunned down in the indeterminate future.

Never underestimate the motivational power of the prospect of being killed. Or of having your friends killed.

We saw it in the Vietnam era, when college students set down their bongo drums and picked up protest signs.

We saw it this past month — in just 30 days — as students from Marjory Stoneman Douglas High School did not merely mourn 17 slain classmates, nor limit themselves to piling teddy bears. Instead they pushed past their inert elders and took on our country’s insane gun culture and the National Rifle Association.

And we’ll see it Wednesday, with the National Student Walkout, when students at thousands of schools leave class for 17 minutes, one minute for every murdered Parkland student. It a litmus test of the mental agility of school administrators whether they embraced this rare moment of youthful solidarity or fought it.

As my colleague Lauren FitzPatrick reported, Chicago Public Schools gave tacit approval. “I want to make sure our students have an opportunity to express themselves and engage thoughtfully in this national dialogue,” CPS CEO Janice Jackson said.

Hundreds of area Catholic schools are holding prayer sessions, discussions and protest activities. Romeoville High School, not content with a single protest, scheduled a week’s worth of activities under the rubric, “It’s Not a Moment, It’s a Movement.”

“As educators, one of our most important tasks is providing students with the guidance of learning advocacy skills for developing policy when they become adults,” wrote James Mitchem, superintendent of Valley View School District 365U.

And then there’s my leafy suburban paradise.

I learned of Northbrook’s cower-in-place reaction when a childhood friend of my son bolted out of his house like it was on fire and buttonholed me. Had I seen the letter District 225 sent out? I had not. He sent it.

After a few paragraphs of throat-clearing, it gets down to business:

“Students who elect to participate in the demonstration will be subject to school rules and attendance procedures as outlined in the Student-Parent Handbook. Walking out of a classroom will be considered an unexcused absence for the period of time that the student is out of the classroom.”

Parents howled. My neighbor, Carla Slawson wrote a letter to Glenbrook North principal John Finan:

“We all want our students to know that there is more to AP Government than getting a “5” on the AP exam … that what they study can and should have relevance in their lives. This is a unique moment to experience student civic engagement in action. Let’s not deny them this, and at the very least, let’s not punish them for it.”

My attitude: what’s wrong with these people? Shame the protest wasn’t a pep rally, because then you could pull everyone of class to have them cheer for Spartan pride. This isn’t about education; it’s about control.

Sunday evening, school administrators met with students who, in keeping with the entire movement, helped the grown-ups grope into the murky abyss of their souls and brush their fingers against their missing sense of moral purpose.

A second letter went out Monday that … again working through the legalese … says, in essence: OK, OK, we aren’t going to punish you for participating in the walkout, much. The gist:

“Public schools must remain politically neutral, and it is through Board policies and school rules that we are able to ensure that the rights of all students are recognized and protected.”

Viewing a desire not to be murdered at your desk as a controversial political stand is political — the politics of the NRA. Compare District 225’s duck-and-cover with the warm, human letter from Loyola Academy principal Kathryn M. Baal, carefully explaining why some students are taking part, exactly what they would be doing, and why the school supports them:

“As a gesture of Ignatian solidarity with the Parkland victims, their families, classmates and friends — and a unified call for change in the way our country addresses school safety, gun violence and adolescent mental health — some Loyola Academy students are planning to participate in the National School Walkout on Wednesday, March 14. … The faculty and staff of Loyola Academy stand behind our students as they grow in their advocacy of peacebuilding and social justice.”

It is hard to run a school. An enormous bell curve of students must be accommodated. It takes rules. I get that. But if administrators aren’t careful, they can get trapped within those rules and forget that rules are not an end in themselves..

Think of it this way:

You’re allowed to run out of the school when a fellow student starts shooting, right? No disciplinary action there. So why not permit students to also leave (for 17 flippin’ minutes!) to join a national effort to kick start the change their parents have failed so woefully to even contemplate? Once again, guns seem to be the default value. You can leave class to flee being shot. But to prevent the next shooting? That’s a problem. To some.