There are literally tens of thousands of unemployed/underemployed military vets who are currently and or have been fully qualified with small arms and basic security and surveillance techniques. After all much of our operations in Iraq and Afghanistan was MP work, Manning check points and doing observational assessment of people they encountered.
Many of these man and women would be more than happy to take on the task of guarding our schools and other designated soft targets/gun free zones. Either on a part time or full time basis. There is absolutely no reason not to use them other than that the Demonrats aren't interested in practical solutions because they don't create exploitable headlines and body counts.