It’s highly unlikely that an armed teacher would be in the right place at the right time to intervene with an armed assailant. And teachers are hired to educate, not shoot people. Many schools have armed resource officers (Miami-Dade has the school police), and having armed teachers is no guarantee that students could be protected from an Adam Lanza or Dylan Klebold.
Additionally, knowing when and whom to shoot in a tense, perhaps chaotic, situation requires training. I'm a high school teacher, and my responsibilities require me to monitor my students.