Late to the debate, but a few "observations".
IMHO, this will add little value. Any "calculating" psycotic(s) (think Columbine), would simply: 1.) Learn the rounds the security guard makes around the school, allowing for an attack when the armed officer is furthest from the target zone. 2.) Kill the armed guard first.
Ok. Correct me if I am wrong but, the 2nd amendment reads: A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed.
Breaking it down. The "Militia" is to protect the "State" aka The USA from "outside" threats. Given elected majority, the Militia is NOT about overthrowing the a/current elected US Government. That would be punishable as Treason under Article III, Section 3. (In 1791 when the 2nd Amed was penned this could have been the British, French, etc. NOT George Washington because you didn't like his wooden teeth).
Also, in 1791 the prevailing weapons technology was; Muscat, Bayonnet, and Cannon's. That said, I say you can own as many Muscats, Bayonnets, and hell- all the 1791 era Cannon's you can store. Restricting AK's, AR's, and high capacity magazines are NOT in conflict.