I've taught school for 33 years. I also own a gun and know how to use it. But it is a ridiculous notion that teachers with guns, or armed guards, will make schools any safer.
There are close to 100,000 schools in the United States. For the cost of hiring 100,000 guards, it would be far more productive to hire behavioral psychologists, who could work with teachers to identify and help students with behavioral and mental problems.
Early identification and treatment would be a win-win-win. These students would lead more productive lives. School classrooms would contend with fewer behavioral problems, and society could avoid the problems these children would create as adults meaning less crime, fewer court cases, fewer people in prison, and more productive, taxpaying citizens.