The idea of arming teachers won't work. Some teachers wouldn't agree to do it in the first place. But even if they did agree, what experience would they have under intense fear and shock? The shooter has the element of surprise. If he can enter a classroom, he can spray the room in a matter of seconds before anyone can react. The teacher would need a couple seconds to get a gun out of a hidden holster or from a desk.
He or she has no experience shooting at a moving target, a human being, with other people in the target area. In his or her excitement or duress, the teacher would just as likely shoot a student. It would take a very experienced or very cool teacher to confront a shooter in a surprise attack. But it would be too late for many of the kids in the room if not the teacher. If the teacher did survive the initial onslaught and he was able to take out the shooter, many kids would already be dead.
I was a high school teacher for 41 years in Orange County, California. I was also in the Marines and the Air Force. I have had some experience with guns. I would probably volunteer for such a program as Trump suggests. But I can't imagine many of the teachers I knew being capable of such an assignment, some of them being on the elderly or physically handicapped side for various reasons. Some couldn't see very well. I can imagine them freezing in disbelief and shock unable to act. Or able to act too late. And some teachers just wouldn't be able in spirit to do it.
No, the idea of arming teachers is a bad idea. We need to stop the relatively quick, easy availability of the semi-automatic assault rifles. The freedom to have guns cannot trump the right to "life, liberty, and the pursuit of happiness."