Heroes: JROTC students' quick thinking, training saved dozens from shooter at Florida high school

Two 17-year-old students at Marjory Stoneman Douglas High School coupled their quick-thinking skills and Junior Reserve Officers' Training Corps training to help save dozens of lives when Nikolas Cruz went on his shooting spree, killing 17 people, Wednesday in Parkland, Florida.

Company commander Capt. Zackary Walls, 17, and Capt. Colton Haab, 17, told ABC News that they had decided to end their outdoor formations early that fateful day. They had taken their students back inside the school just moments before the fire alarm that sounded just as the massacre began.

Walls and Haab took charge and herded students and teachers into a classroom after they heard gunshots ringing out nearby. The two JROTC captains used materials inside the room to create a barrier between them and the killer and prepared themselves for a fight.

What happened?

Walls and Haab were inside the school with their cadets when chaos broke out.

"Around 2:30 I hear the bell ring for the fire alarm, and we start heading out," Walls said. "I'm leading the line, and we're heading out and it just so happens our fire zone was exactly where the shooter was."

Then Walls said he heard the first "pops."

He quickly realized there was an active shooter on campus, although he never saw him.

"I knew it was gunshots and I look back at all the kids behind me, there's 60 kids looking at me [asking], 'What do I do, where do I go' I just yell, 'get back in the classroom,'" Walls said.

Walls, who participates in the Police Explorers program with the Coral Springs Police Department, added that just weeks earlier he had undergone active-shooter training that taught him that a shooter often pulls the fire alarm to get students into the hallway.

Walls said students were running through the hallways, "trampling over each other," and he knew he had to slow down and do something to protect as many as he could.

"I start trying to just herd kids in there, get them to where they're not pushing and trampling each other and just get them into the room safely. I pulled in teachers, I pulled in kids that weren't in my class," Walls explained.

Haab said he heard seven gunshots. He alerted his first sergeant, who told him not to lead his class outside.

What did the students do?

"I shut my door, pulled a student in, and I brought him into the other room [where Walls was located], and I started getting people in," Haab said.

Haab soon noticed the large Kevlar curtains in the room, and he knew what they needed to do.

"We made a wall in front of all the kids out of the Kevlar pads," Walls explained.

"I brought those curtains out because I knew exactly what they were made of," Haab said. "As I'm building them, I'm thinking I would never need this other than what we're going to do, and after yesterday I'm glad we had them."

Next, they took two large tables and barricaded the door.

They grabbed some wood planks, situated themselves on either side of the door and held them across the barricade.

Another student sat armed with a fire extinguisher.

The students were "ready to do what we had to do if someone came in the room," Walls said.