Answers to school shootings aren't simple, but everyone wants to put forth nothing but simple answers to such a complex problem.
The answer isn't asking kids to be nice to one another.
The answer isn't gun control.
The answer isn't arming teachers.
The problem is culture. The really big problem is that it literally took decades to create the culture that produces young people whose belief is that their only opportunity to matter is in destroying lives.
We have a culture that glorifies money.
We have a culture that glorifies fame (or infamy).
We have a culture that glorifies sexuality.
We have a culture that celebrates violence.
We have a culture that focuses on 'me first'.
We have a culture that denies the virtue of strong family bonds.
We have a culture that has abdicated responsibility for the self to outside entities (government, police, etc.)
That's not to say all of us are perpetuating the cultural issues listed above, but those who do are very vocal about it and have the backing of some very powerful entities (the media and entertainment).
Kids whose identities are formed within that culture seek to be accepted by it. When they cannot find their place in the culture that pervades their lives, they force the issue. The Va. Tech shooter wanted vengeance against a girl with whom he was infatuated (sexual failure) and then the infamy of killing a bunch of people on his way out (like his idols from Columbine). The Isla Vista shooter recorded his manifesto in which he complained about hating women and the pain of being a virgin at 22. The Parkland shooter was fascinated by violence. The Sandy Hook shooter had family issues and believed that society wanted him to conform to a set of values he didn't want to embrace.
The list goes on.
Nevertheless, until we can begin to reverse the perverted culture that plagues us all, we will continue to see the scourge of school shootings. The best we can do is to better prepare the first responders in these types of emergencies. The first responders, by the way, are not police, EMS, or SWAT. The true first responders are teachers and students. They need the training. When the average response time for police is 8 minutes, we need to make sure that those 8 minutes don't result in a massive body count.