By the end of February, the students at Manatee School for the Arts in Palmetto, Fla., will see two combat veterans in body armor roaming the grounds, each carrying a 9-millimeter Glock handgun and a semiautomatic rifle with a 17-inch barrel.

If an armed intruder were to enter the campus, “we’re not looking for a fair fight,” Bill Jones, the principal, said in an interview. “We’re looking at an overwhelming advantage.”

Legislation passed after the Parkland massacre last year requires schools in Florida to have at least one “safe-school officer.” Manatee School for the Arts, a charter for 2,100 middle and high school students, is the only school in Manatee County that chose to hire guards who carry rifles, according to the Manatee County Sheriff’s Department, which trained the guards.

[School safety. Gun control. Red flag laws. Read more on howAmerica has responded to the Parkland massacre over the past year.]

“It’s just a much more effective weapon than the handgun is,” Dr. Jones said.

The decision to patrol with long guns is “very unusual,” said Michael Dorn, the executive director of Safe Havens International, which has performed security assessments at dozens of public school systems throughout Florida, including three of the state’s largest school districts.

Mr. Dorn said he was not aware of any school guards in the United States who openly carry long guns, though it is not unusual in some areas overseas.

“It’s not something that we typically advise our clients to do for a variety of reasons,” he added, in part because someone might knock out the officer and take the weapon, and it’s more difficult to subdue and handcuff an assailant while carrying a long gun.

Manatee School for the Arts is also adding a guard shack to its entrance and will raise its perimeter fencing by two feet.

Florida’s bill, named the Marjory Stoneman Douglas High School Public Safety Act after the school where the Parkland shooting occurred a year ago this week, gave schools the option of choosing between a school resource officer, who is employed by a law enforcement agency; a school safety officer, who is employed by either a law enforcement agency or by the district school board; or a school guardian, a school employee trained to stop lethal attacks.

[Read how the Parkland massacre haschanged the lives of the survivors in their own words.]

And in 2016, a teacher in Michigan was grazed in the neck by a bullet when a school resource officer negligently fired his gun. And in some cases, agitated students have grabbed officers’ guns, even while the guns were in holsters.

Dr. Jones estimated that the total cost of the new security measures would be close to $200,000 for the 2018-19 school year, and said the armed guards would earn a yearly salary of about $50,000. It “substantially exceeds” the money received from the state, he added.

Despite the heightened fear over school shootings, active shooters are statistically rare.

Unintentional injury, which includes traffic accidents and drowning, is the leading cause of death for American school-age children.

[Nearly 1,200 children have been killed by gun violence since the shooting in Parkland.Read how teenage journalists told their stories.]

“Our mind-set is so locked into the rare but truly catastrophic active-shooter event,” Mr. Dorn said.

On school property, students and school employees are more likely to die by suicide than a mass shooting, he added.

Spending a lot of money to arm security officers addresses only one type of school safety concern, said Amanda Klinger, the director of operations at the Educator’s School Safety Network, an organization that trains educators and administrators in violence-prevention measures.

“We don’t know that those security interventions will stop the things that they’re seeking to stop,” Ms. Klinger said. “My question is, what else are they doing to keep kids safe in schools? What are they doing to keep kids safe from risks or threats other than active shooters?”