Any person that threatens another person with deadly force would risk death in the world framed by our founders. Because an armed citizenry would pose such a threat to anyone taking the risk to commit armed violence, such a discouraging factor would limit such violence. Likewise, any person harboring such homicidal thoughts would more likely seek therapy rather than risk death.
For instance, 22-year-old Jacob Tyler Roberts who started a shooting rampage in the Clackamas, Oregon Town Center mall, noticed a man with a gun trained on him. Nick Meli had his gun trained on Roberts, but couldn't get a clear shot without endangering other bystanders. He did stop Robert's shooting. In fact Roberts, knowing death from Meli's gun was certain, ran off and found a hall and steps where he shot himself to death.
In Israel, teachers and parents who serve as school aids are armed with semi-automatic firearms whenever they are on school grounds.
Let the teachers unions pay for the weapons (yes, good weapons are expensive) and the extra guards out of the dues money.