While what you say is true, you aren't really suggesting that such people don't kill their students because it is illegal to do so, do you?
Most teachers don't face metal detectors, do they? There is nothing stopping any given teacher from doing exactly the same thing that Adam Lanza did. That it doesn't happen is simply an indication of just how abnormal the shooters are and how essential it is to arm the teachers.
The goal of being armed for self-defense isn't to eliminate all attacks; that is simply unrealistic. The goal is to deter many attacks and to greatly increase the likelihood of surviving such a deadly attack.
I suppose in a way you are right. If there was one crazy teacher, The other (armed) Teachers would be able to stop them. I can’t help thinking “is this what we’ve come to?”