The U.S. murder rate is approximately 5.9 per 100,000 annually. ( See http://www.fbi.gov/ucr/05cius/data/table_16.html ) The U.S. suicide rate is about 11 per 100,000 annually ( see http://www.suicidology.org/displaycommon.cfm?an=1&subarticlenbr=21 ). Guns are used in 68% of murders and 60% of suicides. Add in gun accidents, legal gun deaths (police killings), and undetermined gun deaths, and about 11 per 100,000 Americans are annually killed by guns. This is a fact.
That said, there are some who believe that guns wielded by the good guys can help squash the bad guys. For example, there is an econometric model which estimates that right-to-carry concealed weapons laws are correlated with a 3.3 percent decrease in violent crime for each percentage increase in gun ownership/concealment. This is a model that may be flawed insofar as its underlying data in a significant strata (large cities) contains no variation in the independent variable that is suggested to be causal (namely, the existence of such a law). Questionable data leads to questionable correlations.
I believe a better way to prevent school shootings and overall violence in schools is to treat depression, mental illness, and drug addiction among students, teach children not to sell or use illegal drugs, have police concentrate on enforcing drug laws, and focus more on gang prevention.
A teacher "packing" is spelled t-r-o-u-b-l-e.