So instead of sending her students into the hallway for the supposed drill, Shanthi “Mrs. V’’ Viswanathan, an Algebra II instructor at Marjory Stoneman Douglas HS, ordered them to the floor in a corner of her classroom — then put paper over the door’s window so no one could see inside, the Sun-Sentinel reported.

Outside, former student Nikolas Cruz allegedly began opening fire with an AR-15 assault rifle. He would eventually kill 17 people, authorities said.

When a SWAT team finally arrived and knocked on her door, Viswanathan still refused to open it up, fearful it was the killer playing a trick on her.