Josh Filler has written a long and fact filled blog posting Are Schools Critical Infrastructure in Need of Protection from Active Shooters? It should stimulate our thinking on the subject and cause us to reflect on the various courses of action that can be taken. Read his blog posting first before proceeding further.
I'll add my two cents here. This is a perfect situation for a professional risk analysis with formulas and outcomes that lays out the facts and eliminates the emotions of these types of events. Generally people don't like facts that disagree with their opinions so I doubt that an academic study of that nature will be undertaken--however, it would be a good PhD thesis to pursue if you are in the risk field. Personally I believe we have not seen the last of school shootings because of the notoriety that is being given to these shooting events.
Low frequency, high consequence events are the hardest to get people to pay attention to and to maintain support for. First let's look at the costs. I can tell you that for a professionally trained armed security officer you are going to pay a contractor about $25.00 an hour (This is for an urban area like the Pacific Northwest). That translates to an annual salary of $52,000. King County Washington has around 600 K-12 schools so the bill there would be around $31.2M a year. Josh identified 139,000 schools in the USA. I'll let you do that math.
The old Federal solution to problems caused by disasters is that Congress passes legislation that funds a program and this goes on for a number of years and then starts to tail off with less and less funding while the resource either goes away or the costs are picked up by state and locals. I can tell you we need to end the era of the Federal government funding local protective measures. The budget and the debt will not allow us to continue on this path for much longer. So who pays for this security? It is logical that state and local schools should be funded by state and local tax dollars.
School boards and state legislatures need to determine if this is a significant enough issue that it needs to be funded through new taxes be they at the state level or through local school levies. The idea of robbing Peter to pay Paul and tell schools to "find the money" is a non-starter. Another telling comment about how we value public education and teachers is that these armed officers would likely make significantly more pay than many, if not most, junior teachers. If you make it a local responsibility there will be few takers in my opinion because every community thinks that "it won't happen here."
A "cheaper" solution that some recommend is arming teachers. Anyone who has professionally carried a weapon in the military or in law enforcement knows the amount of training required to have someone become proficient and to maintain that proficiency. Guns in the hands of untrained civilians is not a good solution.
For me there are no single prevention solutions. We have dug ourselves into a hole and in my mind we will keep digging. My short list for "reducing" gun violence is: