Teachers with guns is certainly not the answer. Although some would say this is more security, you must put on your doomsday cap and run through the potential devasting scenarios this could bring about.
The teacher is not an individual trained in personal defense with a firearm. If an intruder would enter the classroom with students and an armed teacher, there are far too many mistakes that could occur. Such a show of force could envoke an even more violent reaction from the intruder.
If the teacher were to fire upon the intruder this would more than likely provoke return fire with the students caught in the crossfire.
The intruder, as in the latest case, has most likely been training for this type of resistence and could disarm the teacher, putting the teacher's and childrens' lives in greater danger and only add to the intruder's firepower.

Trained, armed police officers, especially those with hostage negotiations training, with the power to arrest on site and use deadly force if necessary should be the first choice for added, armed security at any public place, particularly schools.