Teachers are being trained to carry guns in classrooms in Colorado in order to protect children as part of a scheme motivated by a school massacre in 2012.
The three-day course, which consists of firearms and medical training, was launched on Tuesday in Weld County.
Seventeen members of staff who "would like to be considered armed first responders" have so far taken part.
The pilot programme will allow volunteers to enter schools with guns under US "concealed carry" laws.
Teachers were taken to a shooting range in Weld County, near Denver, where they were tested on their abilities with weapons.
The course, provided by the Faculty Administrator Safety Training and Emergency Response group (Faster), was set up by parents, law enforcement officers and medical experts who believe that US schools are a "soft target" for violence. The group refers to schools as "victim zones".
Faster's aim, it says, is not to replace police or security services, but to "allow teachers, administrators, and other personnel to stop school violence rapidly" and to "administer medical aid where necessary".
The group was set up following the Sandy Hook shooting massacre in December 2012, in which a rifle-wielding man killed 26 people, mostly young children, at a Connecticut primary school.
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But the move has angered campaign groups such as Safe Campus Colorado, which encourages its supporters to petition local authorities to help keep guns out of school.
"We believe concealed guns on Colorado's great university, college and community college campuses threaten the safety of students, faculty, staff and administrators," the group says on its website.
Safe Campus Colorado was set up by political activist Ken Toltz, who said in a statement to Colorado's 9 News channel that the move to train staff in the use of guns on campus was detrimental to the safety of both students and teachers.
"The dangers of adding guns to a school environment are dramatically increased by allowing loaded lethal weapons into a school environment on a daily basis," he said.
How does Faster work?
A selected school will ask staff to volunteer for training "in armed response" and "crisis management".
The school will then apply for training with Faster, a non-profit group that relies on charitable contributions, which will provide tuition for free.
Those chosen to participate attend an intensive course over three days with law enforcement personnel experienced in "active shooter situations". They will also be "tested physically and mentally".
The course not only teaches those enrolled to use firearms effectively against any threat, but also how to respond to injuries and provide medical assistance while awaiting emergency services.
An Ohio law issued in 2013 permits schools in the US state to arm employees "if that teacher is required to, in essence, become a security guard".