(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.

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