A "gun free" school is nothing but a "magnet" for mass shooters, President Donald Trump said Thursday, doubling down on a proposal 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.

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

"I will be strongly pushing Comprehensive Background Checks with an emphasis on Mental Health," Trump tweeted Thursday.

"Raise age to 21 and end sale of Bump Stocks! Congress is in a mood to finally do something on this issue - I hope!"

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.

"Bump stocks" are legal 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.