(CNN) When Melissa Falkowski heard there was an active shooter on campus, her instincts kicked in.

"I managed to put 19 kids in the closet with me," said Falkowski, a teacher at Marjory Stoneman Douglas High School. "This is the worst nightmare that could ever happen to you."

A fire alarm had sounded Wednesday afternoon prompting Falkowski to escort students out of the building, just as she had done many times during fire drills. Minutes later, when a security guard alerted her that someone opened fire at the Broward County school , she urged students out in the hallways to hide in the classrooms.

There was a shooter on campus and there was no time -- that's all she knew.

About 60 to 90 seconds later, she closed the classroom door.

Read More