There are several legal and ethical implications for educators when looking at school shooting preventions and reactions.  On the legal side, politicians and lawmakers have spent considerable time and effort looking into what measures can be put into place to make schools safer.  One method includes federal programs to improve preventative education which teachers and counselors will have to then implement. Another  method is arming teachers to defend their classrooms.  If a teacher is able to be armed, what then are the rules of engagement with shooting a child who brings a gun to school with the intent to shoot? Do you wait for them to shoot the first student while you talk them down? Does the age of the child matter in how the situation is handled?  There have been discussions of only arming teachers who were prior military or law enforcement as well as making it optional for teachers who have a license to carry to do so. There is also the issue that one teacher in a school cannot be everywhere at once, if their class is not the target, then they won't be able to respond. 

References:

Kamenetz, A., (2018). Here's how to prevent the next school shooting, experts say. Retrieved from: https://www.npr.org/sections/ed/2018/03/07/590877717/experts-say-here-s-how-to-prevent-the-next-school-shooting
Sandy Hook Advisory Commission (2015). Final report of the Sandy Hook advisory commission. Retrieved from: https://schoolshooters.info/prevention