President Donald Trump has proposed a solution to end classroom massacres once and for all: Arm some of America's teachers with concealed weapons, and train them to "immediately fire back if a savage sicko came to a school with bad intentions," he said Thursday.

But gun violence experts, educators, and school safety advocates immediately panned the idea.

"It's a crazy proposal," said Dr. David Hemenway, a professor of health policy at Harvard School of Public Health and an expert on the public health impact of gun violence. Chuckling, he added, "So what should we do about reducing airline hijacking? Give all the passengers guns as they walk on?"

Avery Gardiner, co-president of the Brady Campaign to Prevent Gun Violence, called it a "colossally stupid idea."

Related: NRA's Wayne LaPierre accuses Democrats of exploiting Parkland tragedy

"If having more guns in more places made Americans safer, then we would have the lowest rates of gun violence in any developed country in the world, and the exact opposite is true," she said, calling the notion that "the only thing that stops a bad guy with a gun is a good guy with a gun" a myth.

"So what should we do about reducing airline hijacking? Give all the passengers guns as they walk on?"

"There could be instances of real confusion that would lead to tragedy if we had more guns in more classrooms," Gardiner said. "What about the time the teacher accidentally leaves the gun unlocked in the desk drawer, and it's picked up by a student? Think about the burden on schools to make sure the teachers are safe to carry guns. Who's doing that checking and monitoring and retraining?"

Statistics show that states with stricter gun laws and fewer guns have less overall gun violence than those with more lax laws and more guns. Hemenway, who has done extensive research on guns, said it boils down to access to weapons.

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"The evidence is overwhelming, starting at the home. A gun in the home increases the risk that people in the home will die. That's because there's more suicides, more gun accidents, and more homicides," he said.

Related: How the internet's conspiracy theorists turned Parkland students into 'crisis actors'

The experts added that even with proper firearms training, to expect a teacher to be able to shoot down an attacker — and not accidentally injure anyone else — is unrealistic.

"To be trained is not just about shooting. Your heart is beating like crazy, your adrenaline is all over your body, and you have to make a wise decision about what to do," Hemenway said.

Brian Levin, a former officer with the New York Police Department, said in the heat of the moment, it's too easy to misfire. He recalled a time early in his career when he almost shot an unarmed man fleeing a shooting scene.

"Often times when you’re having an adrenaline-filled situation, you’re not sure who the target is," said Levin, who is now a criminal justice professor at California State University, San Bernardino.

Dr. Amy Klinger, director of programs and co-founder of the Educator's School Safety Network, a nonprofit that supports safer schools, said she found it "ironic and sad" that the only training being discussed for teachers is weapons training.

"How about training in violence prevention, or all of the other threats that schools face, like severe weather, noncustodial parent fights, and bus accidents?" she asked. "I'm thrilled that the president is having this conversation about school safety. But I'm concerned that we're not looking at the really effective options first."

Not everyone is opposed to arming schoolteachers: In a Washington Post-ABC News poll out this week, 42 percent of Americans said allowing teachers to carry guns could have deterred the shooting at Marjory Stoneman Douglas High School in Parkland, Florida, last week.

And after the Sandy Hook shooting, about 200 teachers in Utah partook in a free gun training course, led by firearm activists who argued that armed teachers could thwart shooting rampages in their schools. Hundreds more in Butler County, Ohio, signed up for a similar class after the Parkland shooting.

Cori Sorensen, a fourth grade teacher from Highland Elementary School in Highland, Utah, receives firearms training with a .357 magnum from personal defense instructor Jim McCarthy in West Valley City, Utah on In this Dec. 27, 2012. Rick Bowmer / AP file

But many prominent figures in education are slamming the idea.

"Educators need to be focused on teaching our students. We need solutions that will keep guns out of the hands of those who want to use them to massacre innocent children and educators. Arming teachers does nothing to prevent that," said Lily Eskelsen Garcia, president of the National Education Association, which represents 3 million educators in the U.S.

Related: How the Parkland students became advocates for gun reform

Even pro-gun educators don't necessarily support Trump's plan. Dr. Joshua Grubbs, an assistant professor of clinical psychology at Bowling Green State University in Ohio, owns multiple guns — but said he couldn't imagine bringing them to class.

"Even under the most ideal conditions, shooting with a handgun is extremely hard. Practically speaking, on a range with ear protection on, at complete peace, that's hard and takes a lot of skills," he said.

More importantly, he said, bringing a gun into class creates an adversarial dynamic.

"You're asking teachers, instead of seeing the best in students, to constantly be on guard for a student that might be a threat," Grubbs said. "It's no longer a safe place for you to learn. It puts the teacher more in a law enforcement role. You relate differently to a teacher than you do to a police officer — and you should relate differently."

Guys, I’m a college professor.



I’ve also been shooting guns my entire life and own multiple guns now.



I don’t trust myself to be able to capably defend a classroom against a shooter, and I’m a good shot.



Why would we expect teachers everywhere to bear this responsibility? — Josh Grubbs (@JoshuaGrubbsPhD) February 21, 2018

In response to questions from reporters about whether it would be practical to arm 700,000 teachers, as Trump has suggested, White House Deputy Press Secretary Raj Shah responded on Thursday afternoon: "I think when you have a horrific situation like you had last week, and some other school shootings that we’ve seen, these horrible tragedies, what we think and don’t think is practical can change."

Fred Guttenberg, whose daughter Jaime was killed in the Parkland shooting, told MSNBC on Thursday that the massacre would have been much worse had teachers been armed.

"You had pandemonium, you had kids running all over, teachers running all over. Everyone was trying to get to a safe place," Guttenberg said. "You would’ve had a shootout, with all these kids and people running all over. That would not have saved lives. It would have led to further loss of life."