That's the latest in the more than 400 school shootings that have happened since Columbine, according to data from the Washington Post. Since then, a multi-billion dollar industry has emerged around school security, and Piers McEnderson takes us on a to see what's for sale.

It's a steamy day in Grapevine, Texas, but that buzzing you hear, it's not mosquitoes. It's a drone.

"We use drones to stop school shootings."

That's Justin Marston. He's the CEO of Campus Guardian Angel, and he's selling these drones. We're outside the National School Safety Conference. Three pilots are doing a demonstration. The drones are whizzing around like tiny helicopters circling a test dummy on a pedestal. He's the shooter in this demo.

The drones are meant to disorient—they can even spit out pepper balls. Eventually they'll ram into the dummy, knocking it to the ground like a bowling pin. This technology is just one example on a long list of things schools can buy to try to deter a mass shooter. Business is booming.

"The school safety and security industry has grown rapidly over the past decade in particular."

Sonali Rajan is with the research arm of Everytown for Gun Safety, which advocates for gun control. According to the market research firm Omdia, the school security industry is now worth as much as $4 billion, and it's projected to keep growing.

[Rajan:] "The challenge right now is that these school safety products, the vast majority of these, have absolutely no evidence guiding their effectiveness."

Inside the school safety conference, there's an expo hall and a lot more for sale. Down the aisles, people are selling panic buttons, bulletproof whiteboards, facial recognition technology, training simulators, body armor, guns, and tasers.

Sarah McNeeley with SAM Medical is selling Stop the Bleed kits.

"Tourniquet, trauma shears."

She says their customers are EMTs, fire departments, military medics, but increasingly: school districts.

Tom McDermott is with CEIA USA, a company that makes metal detectors. He says schools used to be a small fraction of their US business. Now they're the majority.

"Which is nuts. You consider the Secret Service uses us, Major League Baseball, NFL, theme parks, they use us. It's not right. I mean, we need to solve this problem. It's good for business, but we don't need to be selling to schools."

Mo Canady leads the National Association of School Resource Officers, the group that organizes this conference. He says the expo hall is just one part of the gathering. They also train school police officers on how to work with kids who have experienced trauma and how to intervene before violence occurs.

"We're asking a lot of that officer."

So he says when a shooting happens, they need whatever tools they can get.

Gun violence experts say simple things like locked doors can make a difference. Authorities say that likely saved many lives last month in Minneapolis, but that doesn't necessarily prevent a shooting.

Jill Peterson, a criminology professor at Hamline University, says people who plan a shooting but don't carry it out often change their minds for two reasons. One is that they can't access a gun. The other is that someone helps them when they're in crisis.

"We're spending billions of dollars that could be going to mental health or counselors."

Still, she says, she understands the allure of an impenetrable school.

"And I think it preys on people's worst fears, right? Like, how do you say no to something if you're telling me it might save my kid's life? Of course I want that thing."

She says trying to buy our way to safety feels very American, just like school shootings.

Meg Anderson, NPR News.