The Gun Control argument is based on the premise "We can't stop school shootings from happening but we can prevent people who don't shoot up schools from legally defending themselves with a gun. We can make it harder for people to get a gun just like we made it harder for people to get Marijuana and Heroin. But the government can't prevent people from getting a gun, just like they can't prevent people from driving illegally. "
The only way to prevent school shootings is to make the shooter think it can't be done. Most likely, the shooter found the locations of the armed people and went to another site away from them. A school shooter knows he has a target rich environment with little chance of preventing the shooting. Schools are created in a way that they are ideal for shooters.