Police say ‘active shooter protocols’ saved students’ lives as a gunman opened fire at a Florida high school, injuring one student in the ankle, in what is now the 20th school shooting in the US this year.

A gunman carrying a shotgun in a guitar case walked into the Forest High School in Ocala about 8.39am on Saturday (Friday local time), just before a national classroom protest was to start to protest gun violence on the anniversary of the 1999 massacre at Colorado’s Columbine High School.

One student, a 17-year-old boy, was wounded and taken to a local hospital for treatment with a non life-threatening injury to his ankle.

Police have praised the quick thinking of students and teachers who piled desks and cabinets against classroom doors to make barricades.

The Marion County Sheriff’s Office (MSCO) said that when deputies arrived on scene, they discovered that MCSO School Resource Deputy Jim Long had taken the shooter, 19-year old Sky Bouche into custody “without incident”.

Police said the suspect, later named as former student Sky Bouche, drove to the school and walked on campus with a shotgun concealed in a guitar case.

“Once inside school grounds, Bouche entered a bathroom and put on a tactical vest and gloves to prepare himself for the shooting…he shot one round from his sawed-off shotgun through a classroom door,” the office said.

He was arrested and apologised as he was led away in handcuffs.

Bouche said “sorry,” followed by “It doesn’t matter anyway” to reporters as he was led from the school in handcuffs by several deputies.

“I didn’t shoot anyone,” he said to reporters.

He ignored most of the other questions until asked what he’d say to the shooting victim. That’s when he said, “sorry.”

MSCO Sheriff Billy Woods said the resource officer’s quick response and active shooter protocols at the school helped save lives, CNN reported.

“It’s a shame what society has come to in that we even have to be here on a school campus,” Woods said.

“Society has changed since I was in school. … We as a whole need to do something. My emotions are running rampant,” he told reporters.

A student was injured in a shooting at Forest High School in Ocala, Florida. The suspect is in custody.

It happened shortly before a planned walkout to join a nationwide protest against school violence today. pic.twitter.com/fxqpwJdvP4

— AJ+ (@ajplus) April 20, 2018

Bouche is charged with terrorism, aggravated assault with a firearm, culpable negligence, carrying a concealed and unlicensed firearm, possessing a firearm on school property, possession of a short-barreled shotgun, interfering with school function and armed trespass on school property.

The shooting comes just over two months after a gunman killed 17 people and wounded 17 others at Marjory Stoneman Douglas High School in Parkland, Florida.

Nikolas Cruz, 19, faces the death penalty if convicted in that Valentine’s Day shooting.

-with AAP