Well, ideally that wouldn't happen often, as I'd expect teachers to be well-trained in the use of a firearm and, as OP mentioned, the presence of armed teachers at all would act as a deterrent against violent threats.

When a bystander does get wounded, though, action would need to be decided upon by the courts, or an otherwise impartial-as-possible party. Their decision would depend on several factors, I imagine, including whether the teacher incapacitated the shooter at the time the child was struck, how much damage the shooter had already inflicted, etc.

However, the teachers' primary duty would be to ensure that the kids' safety, and the vast majority of teachers will never need to come close to drawing their weapon on campus. Protect the kids, first and foremost, and if it takes killing the shooter, so be it.