Adam Lanza was wearing a bulletproof vest. How could armed teachers have stopped him?
Or rather, for that matter, armed security guards or even a single police officer?
http://www.usnews.com/news/articles/2012/12/20/the-other-loophole-bulletproof-armor-newtown-guns
"Adam Lanza wore a bulletproof vest as he opened fire at Sandy Hook Elementary School in Newtown, Conn., last Friday, killing 26 people20 of them children under the age of 10. James Holmes, the alleged perpetrator of the Aurora, Col., theater massacre in July, wore an armor vest and a ballistic helmet."
Ordinary citizens with concealed guns are no match for a gunman with a bulletproof vest, and even armed security guards and a single police officer are no match for a gunman with bulletproof vests and military grade machine guns, and imagine if gunmen load their guns with armor piercing bullets (as in the case of the bank robbers in the 1998 North Hollywood Shootout.)