Students decry gun violence outside the White House on Feb. 21, 2018. (Alex Wong/Getty Images)

Turned out to be nothing, the full-alert police swarm on a Maryland middle school, and the kids sheltering in place most of Wednesday morning.

But the threat that gun=shooter=massacre is not make-believe.

“As a mom, you can’t tell them, ‘Oh, this is never going to happen here,’ ” said Kathleen Isaacson, whose eighth-grader was in class at Silver Spring International Middle School when the school received a report of a gun. “Because that would be a big, fat lie. It has happened. Kids have seen that these things happen.”

At Silver Spring International, Isaacson’s child and about 1,000 others knew what to do: They hunkered down in their locked classrooms. On Thursday, the target was nearby Albert Einstein High School, where a bomb threat led to a lockdown for 1,700 students. A helicopter circled as bomb-sniffing dogs arrived with their handlers to investigate.

This is the awful, unnerving reality with which our children are growing up. There have been a rash of threats across the country since a gunman with AR-15 semiautomatic rifle walked into Marjory Stoneman Douglas High School in Parkland, Fla., on Valentine’s Day and slaughtered 17 people.

Ordinarily, according to the Educator’s School Safety Network, about 10 threats or violent incidents are reported each day in American schools. Since Parkland, the network, which has been tracking these incidents for years, said reports have increased sevenfold. The network is recording about 70 incidents every day — 673 since the Parkland shooting.

Active-shooter drills are part of our kids’ education. They learn to hide in closets and stay silent while an imaginary gunman prowls the halls. Some elementary schools tell their students to pretend they are hiding from a deadly tiger.

Millions of kids have not lived through a school shooting but are filled with fear that they will.

The NRA/AARP Venn diagram crowd will dismiss the idea that the drills themselves are traumatizing: “Come on, we faced down nuclear annihilation when we did duck-and-cover drills as kids! They’ll be fine!”

Hold your horses, Boomer Boy. The threat of nuclear annihilation was coming from an enemy an ocean away. No school was ever blown up.

Today, kids expect the worst because they’ve seen it again and again.

In Silver Spring, the students weren’t even told about the gun threat. They figured it out after reading social media and seeing reports, and some texted goodbye messages to frantic parents. Some sat stone-faced at dinner that night.

Isaacson’s eighth-grader even asked her mom to stay in her room while she fell asleep Wednesday night, a return to toddler years. Other parents told Isaacson that their kids did the same.

My son’s school hasn’t had a lockdown, but it recently conducted an intense active-shooter drill. The next weekend, my son ducked for cover in the laundry room when he heard a loud noise outside. That night, he slept on the floor of our bedroom. He’s 11.

Last week, someone tweeted, “2/22/18 I am coming and no one can stop me be prepared . . . ,” and then, “Fairfax high school is some [expletives] i hate that damn school so much,” then explained, using more foul language, that he was suspended and “I have been planning this for months now and now I am going to act. Better watch out fairfax . . . ”

Alarmed folks collected his tweets and told police, while parents debated online whether to send their children to school.

Police added security at the school that day and investigated the threat.

It has become routine. And yes, go ahead, Gen Xers — remind us how someone called in a bomb threat every time midterms came around.

But were schools actually blown up? No, not since the one in Bath Township, Mich., in 1927, where 45 people were killed. Horrible, but it hasn’t happened again in 90 years.

The shootings are different. They’re real, and they’re relentless.

More than 150,000 students attending at least 170 primary or secondary schools have experienced a shooting on campus since the Columbine High School massacre in 1999, according to an ongoing and conservative Washington Post analysis that does not include after-school assaults, accidents and suicides that involve guns.

So our nation runs kids through regular active-shooter drills, and we’re talking about arming teachers and fortifying school buildings with metal detectors, moats, smoke cannons and bulletproof glass.

What about parks, health clinics, movie theaters and shopping malls? Is the Cinnabon cashier at the mall or the ticket taker at the multiplex going to have to carry a weapon?

Our nation will not be safe unless we get to the heart of the problem. We all know what that is: guns.

Twitter: @petulad