WASHINGTON, Feb. 23 (Xinhua) — A "gun free" school is nothing but a "magnet" for criminals, US President Donald Trump said Thursday, a day after proposing to train and arm some teachers to keep US schools safe.

"Highly trained, gun adept, teachers/coaches would solve the problem instantly, before police arrive. GREAT DETERRENT!" Trump tweeted.

At a White House meeting late Wednesday with survivors of a shooting rampage at a Florida high school that killed 17 people, Trump suggested arming a select group of teachers to deter mass shootings.

"I never said 'give teachers guns'... What I said was to look at the possibility of giving 'concealed guns to gun adept teachers with military or special training experience-only the best. 20% of teachers, a lot, would now be able to ...immediately fire back if a savage sicko came to a school with bad intentions. Highly trained teachers would also serve as a deterrent to the cowards that do this. Far more assets at much less cost than guards. A 'gun free' school is a magnet for bad people. ATTACKS WOULD END!"

At the Wednesday meeting, billed as a "listening session," Trump promised "very strong" background checks on gun owners.

He endorsed a National Rifle Association (NRA) proposal to arm teachers as a way of preventing massacres like last week's mass shooting at a Florida high school.

Trump also had an emotional hour-long White House meeting with students who survived the Florida shooting and a parent whose child did not.

Hundreds of students joined scattered protests across the country on Wednesday, including in Washington, Chicago and Pittsburgh.

The Republican president, who has championed gun rights and was endorsed by the NRA during the 2016 campaign, said he would move quickly to tighten background checks for gun buyers and would consider raising the age for buying certain types of guns.

Trump spoke at length about how armed teachers and security guards could frighten off potential school shooters and prevent student deaths. "If you had a teacher... who was adept at firearms, it could very well end the attack very quickly," he said.

The attack in Parkland, Florida, where 17 students and educators were slain on Feb. 14 by a gunman with an AR-15 semi-automatic assault rifle in the second-deadliest shooting at a US public school, has revived the long-running US debate over gun rights.

The US Constitution protects the right of Americans to bear arms, a right fiercely defended by Republicans. But Trump has been under pressure to act.

Andrew Pollack, whose daughter Meadow Pollack, 18, was killed, shouted: "It should have been one school shooting and we should have fixed it. And I'm pissed-because my daughter-I'm not going to see again."

Trump sat in the middle of a semi-circle in the White House State Dining Room. Photographers captured images of his handwritten note card with questions like "What would you most want me to know about your experience?" and "I hear you."