When a gunman opened fire outside Stephanie Reyna’s classroom at Apalachee High School in Georgia, killing math teacher and football coach Richard Aspinwall, her classmates jumped into action, locking and barricading the door.

The chaos began when Reyna, 17, said her class “heard banging on the lockers right outside the classroom door.”

Brayan Maldonado, also 17, said it sounded like someone being pushed against a locker.

“My teacher, Coach Aspinwall, opened the door and ran out to see what was going on,” Reyna told ABC News.

“We heard some popping noises,” Reyna said. “We just stopped, we froze, we didn’t know what was going on. … So we all ran to the back of the classroom. We hid in the corner.”

Reyna said she and her 17 classmates were on the floor for several minutes when they heard more pops.

“Then we realized our classroom door was still open,” Reyna said.

Maldonado said he heard “a little bit of breathing” and “a little bit of moaning.”

The students then saw Aspinwall lying on the ground, they said.

“He was just laying there, in the doorway,” Reyna said. “He was trying to crawl back to us … we think he was just trying to get to us.”

“A few minutes pass. He’s taking a breath,” Maldonado said. “And then we hear his last breath.”

“Then one of my classmates had the courage to get up from hiding” and drag Aspinwall’s body into their classroom, Maldonado said.

“That gave me the courage to stand up,” Maldonado said.

He said he and his classmates closed the door and barricaded it with cabinets, desks and chairs.

“We did everything we could to make sure (the shooter) couldn’t get in,” Maldonado said.

Once the door was locked, Reyna and Maldonado tried to comfort their classmates.

“Some were hyperventilating, some were crying,” Maldonado said. “I was really trying to calm everyone down. When everyone calmed down, I calmed down and all the feelings came out.”

Reyna said emergency workers evacuated the students to another classroom before joining the rest of the school on the soccer field.

Aspinwall, fellow teacher Christina Irimie and two students were killed in the shooting in Winder on Wednesday morning. Nine others were wounded.

The 14-year-old suspect, Colt Gray, a student at the school, surrendered to school police at the scene and was arrested, authorities said.

Gray is charged with four counts of murder and will be tried as an adult, authorities said. A motive has not been released.