When Donald Trump proposed arming teachers to deter school shooters, the overwhelming response from educators was horror. Yet teachers in Colorado are taking action to carry concealed guns to school.

The non-profit group FASTER, set up after 20 small children were killed in 2012 at Sandy Hook Elementary School, has trained more than 1,300 US school staff, mostly in Ohio, on how to use a handgun in the event of a school shooting.

Sixty-three of them have been trained in Colorado, home to the 1999 Columbine High School shooting. The swing state has a Democratic governor and both a Democrat and a Republican in the US Senate.

"I think it's scary to people to bring a weapon into school," conceded Katie, a first-grade teacher in Jefferson County, west of state capital Denver, who declined to give her second name.

"They just see the bad side, not the positive side of how guns can save people," the 27-year-old said.

She was a student this week at the three-day, $1,000 course in Commerce City outside Denver.