I apologize if this post sounds insensitive to anyone, but i do realize that we're going to have to look at the economic costs of any additional security measures that we may implement.
As some others have already mentioned, deaths in mass shootings are insignificantly rare from a statistical standpoint. That doesn't mean that we shouldn't try to prevent these events, but it does perhaps suggest that we need to be cognizant of the fact that we can't invest hundreds of billions of dollars to stop these incidents, when empirical evidence suggests that this money could be better used to ensure the safety of our citizens in areas that are costing far more lives each year.
I'm a police officer, and I do fully support the idea of having a full-time police officer in every school, at least when we're looking at this issue from a purely security-related standpoint. But, I also realize that staffing schools in that manner is not going to be cheap, and probably isn't the only answer to consider in every case. As such, I do support the implementation of a structured program to appropriately train and arm interested teachers in each school. I don't see why we couldn't pay these teachers a few extra dollars an hour for their specialized expertise, and doing so would still be cheaper than having a dedicated armed security employee, and would provide more security than you could expect by having no one in the school armed.