> Is it true that a teacher can't engage/attack a shooter from behind
> during a school shooting that is in progress?

No. You can't shoot someone who is not an ongoing threat to others if they flee, but under the leading constitutional case (*[Tennessee v. Garner][1]*), this limitation doesn't apply to murderers and mass shooters anyway.


  [1]: https://supreme.justia.com/cases/federal/us/471/1/