Marjory Stoneman Douglas High School in Parkland, Florida, had a plan and security measures in place in case of an active shooter situation.
"We could not have been more prepared for this situation, which is what makes it so frustrating," said teacher Melissa Falkowski.
On Wednesday, a gunman entered the school and started shooting, killing 17 people. Falkowski said they did everything they were supposed to do, and she said their training saved lives.
The shooter is in custody and has been charged with 17 counts of premeditated murder.
Additional reporting from Newsy affiliate CNN.