Trump insisted he was not advocating arming every American teacher, but only those with "military or special training" -- suggesting that would be around 20 percent.
That would mean weapons for around 700,000 educators, a potentially massive business opportunity for gun manufacturers.
He said they'd be "comfortable" with it, and that a would-be shooter would be deterred.
"They're not going to walk into a school if 20 per cent of the teachers have guns. It may be 10 per cent, it may be 40 per cent," he said at the White House.
"And what I'd recommend doing is the people that do carry, we give them a bonus. We give them a little bit of a bonus. Frankly, they'd feel more comfortable having the gun anyway. But you give them a little bit of a bonus. So practically for free, you have now made the school into a hardened target."
Trump also declared "gun free" schools a "magnet" for mass shooters.
"A gun-free zone to a killer or somebody who wants to be a killer, that's like going in for the ice cream," he said.
"That's like, 'Here I am, take me.'"
Trump also proposed raising from 18 to 21 the minimum age to buy more guns than at present -- like the assault-style rifle used by 19-year-old Florida shooter Nikolas Cruz -- and making it more difficult for the mentally ill to own firearms.
Currently under federal law, anyone 18 or over can buy a gun from a private, unlicensed seller, although a handful of states have set the minimum age at 21.
Those measures, which may struggle to pass the Republican-controlled Congress, could have put him at odds with the NRA, which donated to and endorsed his campaign.
"I really think the NRA wants to do what's right," Trump said.
"I mean, they're very close to me, I'm very close to them, they're very, very great people. They love this country. They're patriots."
Teachers' unions were quick to condemn Trump's proposal, with the American Federation of Teachers claiming Trump was in favor of an "arms race" that would "turn schools into militarised fortresses."
"Anyone who wants guns in schools has no understanding of what goes on inside them—or worse, doesn't care," the union's president Randi Weingarten said.
Summing up the opposition of many lawmakers, Democratic Senator Richard Blumenthal called the idea of arming teachers "toxic lunacy -- an NRA-backed distraction from common sense action.
Tyra Hemans, 19, left, and Logan Locke, 17, right, student survivors from Marjory Stoneman Douglas High School wait to board buses in Parkland. Source: AP
"Arming teachers is inane and insane -- a sure path to reckless and panicky shooting, gun theft, and other deadly dangers. A non-starter in the Senate."
It comes as National Rifle Association chief Wayne LaPierre hit back at what he called "the shameful politicisation of tragedy," fighting a rearguard defense against street protests and mounting demands to tighten America's permissive gun laws.
In his first public comments since the Valentine's Day rampage that left 17 dead in a Florida high school, LaPierre reiterated long-standing accusations that gun control advocates were seeking to roll back the constitutional right to bear arms.
"It's a classic strategy right out of the playbook of a poisonous movement," he told an annual conservative conference outside Washington, hitting out at supposed "socialists" on the political left, and at the "so-called national news media."
"For them, it's not a safety issue, it's a political issue," he charged. "They hate the NRA. They hate the Second Amendment. They hate individual freedom."
He also doubled down on the NRA's longstanding position that armed Americans were the first line of defense in confronting deadly attacks, saying: "To stop a bad guy with a gun, it takes a good guy with a gun."
The NRA's cause received a significant boost when Trump -- in his second meeting at the White House on school safety in as many days -- doubled down on a plan to respond to the Parkland carnage by putting more guns in schools.
'Mood to finally do something'
On Wednesday, Trump heard wrenching first-hand accounts from bereaved parents and friends, and teens from Parkland who narrowly escaped with their own lives on February 14.
The US Congress has long been deadlocked on the gun debate, accomplishing nothing despite a spate of mass shootings and polls showing that Americans support stricter gun laws by a two-to-one margin.
According to a Gallup tracking poll, 60 percent of Americans now favor tougher gun sale laws.
Trump has also advocated banning devices that clip on to the back end of a rifle and harness the gun's recoil, essentially turning a semi-automatic weapon into an automatic one.
"Raise age to 21 and end sale of Bump Stocks!" he tweeted earlier Thursday.
"Congress is in a mood to finally do something on this issue - I hope!"
