2265 Legitimate defense can be not only a right but a grave duty for one who is responsible for the lives of others. The defense of the common good requires that an unjust aggressor be rendered unable to cause harm. For this reason, those who legitimately hold authority also have the right to use arms to repel aggressors against the civil community entrusted to their responsibility.If a teacher FAILS to be properly armed and ready to shoot an intruder dead, has that teacher committed a mortal sin?
Hmmm... Grave matter, full knowledge, refusal to do as Christ commands - quite possibly "yes."
I can choose to be a martyr myself, but I cannot choose, by my action or inaction, to make someone else a martyr. I cannot refuse to arm myself because I don't like the idea that I might have to use deadly force to protect the ones who have been placed in my care.
Now, when Cardinal O'Malley called for "more gun laws", perhaps he really did mean to say that all teachers should be trained in deadly force.
If that wasn't what he meant, he may want to read the Catechism.