In some states, non-active LEOs (Law Enforcement Officers) would be prevented by law from being armed, especially as private citizens on school property. H.R.218 is not widely accepted enough to provide for this (
http://en.wikipedia.org/wiki/Law_Enforcement_Officers_Safety...)
In my state many middle and high schools have assigned SROs (School Resource Officers / Deputies), whose primary role is not protection, but to work with students. The secondary benefit is the presence an armed, actively trained LEO who could respond to an active shooter.
While many would decry the costs of adding these SROs to every Elementary - High School, the cost would likely be insignificant (in terms of money and effect) to turning schools into secured facilities (with prison like barriers).
http://www.tricities.com/news/local/article_a66f42d4-4806-11...