A California man has joined the growing ranks of entrepreneurs pushing bulletproof backpacks as a solution to increasingly bloody school shootings.

But experts are skeptical that the armor would work in any but the luckiest circumstances for the kid wearing it. They're equally skeptical that the psychological cost of turning schoolkids into soldiers is worth the off chance that a slab of armor might stop a bullet.

"There is no serious research that would support this approach as a practical measure to save lives in a school shooting," Matthew Mayer, a Rutgers professor and school-shooting expert, told The Daily Beast.

Matt Materazo, owner of Danville, California-based Gladiator Solutions, has begun offering an 18.5-ounce, quarter-inch-thick armor insert that he claimed can deflect a bullet from an AK-47 or equally powerful rifle or handgun. “PakProtect fits perfectly into the standard backpack, providing light and unobtrusive protection,” Materazo told The Daily Beast.

The PakProtect armor plates, made of strong polyethylene plastic, slip inside any normal backpack like a student or worker might carry. "The student and/or business person would keep their backpack, with their PakProtect armor insert, with them at all times," Materazo said.

Materazo said he doesn't mean for the plate to solve the problem of school shootings. It is, however, a "terrific compliment" to the "run-hide-defend" strategy the Department of Homeland Security recommends in the event someone opens fire on a crowd or school, Materazo said.

As its name implies, the "run-hide-defend" strategy calls for targets of mass-shooters to try fleeing. If they can't, they should hide. Only if that fails should they attempt to fight back.

Materazo's plate, which retails for $130 on his website, in theory is proof against an AK-47's 7-.62-millimeter-diameter round—although Materazo admitted the impact of the bullet on the plate would knock a child onto the ground.

But the 11-inch-by-14-inch plate isn't big enough to cover all of a kid's vital organs, according to Lynn Westover, a former Marine sniper who is now a security consultant with Six Layer Concepts & Consulting based in Washington State. "Even on a little kid, that doesn't cover a whole lot."

There are other complicating factors that make the armor impractical. "Are they wearing it properly?" Westover asked. The plate would need to be perpendicular to the bullet's path in order to work reliably. "If they're running left or right, they don't have any protection on them."

A schoolkid would need to have their armored backpack with them at all times—and be prepared to don it, and move in such as way as to maximize the plate's effectiveness, all while under fire. "You'd have to drill your child with it," Westover said. "I'm curious how you're going to explain this to your child."

There's a cost in equipping and training a schoolkid like you would a soldier. "Widespread and ongoing use of armored backpacks could increase general anxiety and fear, possibly leading to problems with academic performance and healthy social relationships," Mayer said. "It could also provide a false sense of security where an individual would not follow their natural self-preservation instincts in a dangerous situation, perhaps feeling invincible."

Westover stressed that, in his time in combat in Iraq and Afghanistan, running was almost always the best defense against gunfire. He added that when it's time to run, you want to be carrying as little as possible.

Leaving aside stricter and more widespread gun control, the best way to protect a school and its pupils isn't necessarily to hang armor on kids' backs, but to nurture trust, openness and alertness among students, parents and teachers, experts said.

Shooters tend to be "people that kids know," Ron Avi Astor, a professor and school-shooting expert at the University of Southern California, told The Daily Beast. Students, parents and teachers often sense something is wrong with a potential shooter long before they pick up a weapon, Astor said.

People need to be sensitive to problems with their peers and pupils—and they need to know who to talk to once they do detect a problem, Astor said. "These backpacks send a different message."

Counting on armor to save lives actually erodes trust and makes it harder to stop a shooter before they pull the trigger.

If anything, Astor said, we should be "softening" schools by decreasing security, not "hardening" them with military-style protections such as armor. Less security would encourage communication — the best defense against a potential shooting, Astor explained.

Besides, Westover said, the normal contents of a backpack could be as effective as a purpose-made armor plate at stopping a bullet. "If you put two inch-and-a-half-thick textbooks and a laptop computer in a backpack, you'd have the same results."