By Nicole Chavez, CNN
(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.
"We sort of huddled in the corner for a few minutes and then I made the decision to move everyone to the closet," Falkowski told CNN's Anderson Cooper.
Most students stood in silence while on their phones, some others burst into tears. Falkowski said she just tried to hold herself together.
"You try to do the best you can for the kids you are supposed to keep safe," she added.
For 30 minutes, the teacher and the students waited in fear until they heard SWAT officers securing the room.
They were safe.
Falkowski credited a recent active shooter training with saving lives. And yet, it didn't save enough.
"We could not have been more prepared for this situation, which is what makes it so frustrating," Falkowski said.
"We did everything that we were supposed to do. Broward County Schools has prepared us for this situation and to still have so many casualties, at least for me, it's very emotional. Because I feel today that our government, our country has failed us and failed our kids and didn't keep us safe."
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