(CNN)Colton Haab heard seven gunshots, and his Junior ROTC training kicked in.
The 17-year-old junior knew a gunman was on his high school campus Wednesday afternoon so he ushered 60 to 70 people to shelter in an open Junior Reserve Officers' Training Corps room. At that point, he realized that the Kevlar sheets generally used for the Junior ROTC marksmanship program could come in handy.
"We took those sheets, and we put them in front of everybody so they weren't seen, because they were behind a solid object and the Kevlar would slow the bullet down," Haab told CNN on Thursday.
"I didn't think it was going to stop it, but it would definitely slow it down to make it from a catastrophic to a lifesaving thing."
Haab's quick thinking was one of a number of heroic responses to Wednesday's mass shooting at Stoneman Douglas High School in Parkland, Florida, in which a gunman killed at least 17 people.
Fortunately, the gunman did not come into the Junior ROTC room, and the Kevlar was not needed. But if he had done so, Haab said he and a friend had a plan.
"I was a little scared. I was more worried about getting home safe, making sure everybody got home safe," he said. "God forbid, if he did come into the classroom. I didn't want that to happen, but if it did, I would try to stop him with another friend of mine that was with us."
Though Haab's heroism wasn't needed, assistant football coach Aaron Feis' was. Haab said he saw Feis running toward the gunman, and he later learned that Feis was among those killed as he tried to shield students from the shooter.
"That's Coach Feis. He wants to make sure everybody is safe before himself," Haab said.
"He's definitely in a better place now. I'm glad that he didn't suffer that much. It's sad because it's not going to be the same without him at school anymore."