I wasn't there.
The teacher has the door blocked.....there's shooting going on. He opens the door to let in a few kids and maybe the shooter gets into the room too and guns down more kids.
He might have made the best of several bad choices available to him. Who knows?
My daughter is student teaching at a large high school.
A kid brought a gun school. Someone reported him, police called, school went on lock-down.
At least one teacher, after initially securing the room, continued to let students in, including kids not in her class.
Turns out she let in the kid they were hunting for. He hid with the other students in her room. The student's name was not given out. After hours of lock-down, the police eventually figured out where he was.