As a retired teacher, this is what we have to do. Get all children into the designated area in the classroom. (Never knew what we would do on the playground) The designated area is one where a shooter cannot shoot the children through a window or a wall. The wall chosen has to be one that bullets cannot penetrate. And the children cannot be huddled together as bullets go through them like butter. Aim for one and the shooter can take a bunch out at one shot.
If there are windows in the classroom, pull all shades down. Of course, step one is to lock the classroom door.
And of course you have to keep the children absolutley silent so the shooter is not alerted to your presence. I feel there is no time for a fire extinguisher. As a teacher, should the teacher make a decision with a gun when trained officers have issues with that?
As a teacher, we look to finding the positive in each child. Blowing one away who may have been a former student or neighbor seems a bit harsh.
Funny these issues happen rarely in other countries.