My impression is that the public schools with a fixed police presence are typically middle and high schools. The primary role of the police in these schools is not to protect the students from an outside threat, such as a would be mass killer, but to handle the range of police problems that occur in the student community...gangs, drugs, theft, truancy, etc.
I am not aware of any elementary schools with SROs, for the issues that SROS typically deal with are not usually present among elementary school kids. They're just too young.
So, placing an officer in a given elementary school to protect against a statistically remote chance of mass murder attempt is hard to justify, from a financial standpoint. The SROs in middle and high schools have much more to deal with to justify their positions...and the cost of those positions.
So, I see the biggest obstacle to placing police in elementary schools as being financial.
Of course, a madman looking to carry out an attack in a school might well choose an elementary school over a middle or high school because it is the softest of soft targets...if Newtown style attacks become a trend, then the financial calculus could change.
I agree that many of the liberal mindset have an aversion to a police presence on schools, any school, and would be similarly averse to armed private security at schools. Again, that aversion is subject to change if Newtown style attacks become the trend.
"The great enemy of the truth is very often not the lie, deliberate, contrived and dishonest, but the myth, persistent, persuasive and unrealistic." President John F. Kennedy