Well, if teachers and students could/would have carried this would not have happened or, if it did, the number of killed and wounded would have been far less. With only one security officer on duty (unarmed) the teachers and students were sheep to the butcher's slaughterhouse.
For example, the Pearl, Mississippi and Edinboro, Pennsylvania school ground shooters were stopped by an armed Asst Principal and an armed citizen respectively. And in a church in Colorado Springs:
The gunman who killed four people in two church-related attacks on Sunday committed suicide after being shot “multiple times” by a volunteer security guard at a church in Colorado Springs, the El Paso County coroner said. The man, Matthew Murray, 24, was confronted by the guard, Jeanne Assam, a former police officer and member of the New Life Church, as he entered the building. He fired a single bullet into himself as he lay wounded, the corner said. Mr. Murray killed two people early Sunday at a missionary training center near Denver and had just killed two teenage sisters in the parking lot at New Life, about 12 hours later, before facing Ms. Assam.
http://www.nytimes.com/2007/12/12/us/12brfs-GUNMANKILLED_BRF.html?fta=y&pagewanted=print&_r=1&
So yes, if more people carried then this sort of mass shooting would either not happen, or been opposed by armed "victims".