[1982-2012] "Of the 139 guns possessed by the killers, more than three quarters were obtained legally." (motherjones.com)
100% of the weapons themselves were legal weapons. The majority of the weapons used are semi-automatic weapons, seconded by assault rifles.
This will not stop such attacks (China's school stabbing harmed 20 kids, only 1 died); however, we can prevent the number of causalities by reducing access to weapons/clips that allow quick and effective murder.
Arming and training 7.2 million (census.gov) public teachers to fight against semi-automatic weapons of suicidal shooters will not stop shooters from killing an entire room of people before some armed teacher can access and load their gun.
As now, owners can keep their guns unless they are charged with a felony.