ARGYLE, Texas (CBS11) – Argyle High School has changed over the past five years.

Only one of four doors at the campus entrance is unlocked.

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But once you walk in, a school staffer must provide clearance for all visitors, while sitting behind a sliding security glass. The added layer of campus access is part of a school district security plan that also includes loaded handguns.

“Our staffers have their weapons on them all the time,” Argyle ISD Police Chief Paul Cairney said Thursday.

On the four campuses that make the Argyle ISD, appointed and trained staffers are designated armed employees.

The district’s Armed Staff Program allows selected teachers to carry concealed handguns, as part of the school district’s security plan. After the 2012 school shooting massacre in Newtown, Connecticut, the killing of 20 children and six adults inside Sandy Hook Elementary School, Argyle ISD debated various methods to enhance school security from active shooter attacks.

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The Argyle ISD Board of Education voted in favor of state-allowed Marshal Plans.

School districts can allow teachers or other employees to bring guns to school. The staffers must be licensed to carry handguns, complete required training, and pass a psychological evaluation in order to be selected for the Argyle ISD program.

“We feel its more effective if a staffer has (the gun) on them always, so they can respond immediately if a situation happens,” Cairney said.

The district added a police force, including Cairney as Chief, three years ago. But the Armed Staff Program is viewed as a safety supplement.

The designated armed educators are not known by students, and after years of the teacher security operation, no shot has been fired.

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“We’ve never had to engage an adversary on campus, which is good. We hope we never have to,” Cairney said.