There are a couple of points that I think are being missed.
I'm not making an argument for or against guns - but to say that everyone carrying guns would help this situation is absurd. Has anyone noticed that with virtually all of these school shootings - the killer(s) kill themselves in the end. In every instance I can think of the shooting spree guys had every intention of dying.
Now how would carrying guns around have prevented this... sure someone might kill him before 33 people but the underlying problem is not addressed. Even if everyone had guns - the kid could surely take out 10 easily before anyone responded... and why would he care if they respond. He planned on dying anyway! Second crazies might just go to other means, bombs, etc. So by saying everyone should carry a gun - you haven't solved the problem of school violence and two you have certainly increased the chances of an accident. Last time I checked drunk college students full of hormones don't make the best gun carriers.
So while you may say carry guns to prevent other crimes - I don't think it applies here at all.
And to those who say Switzerland has guns - BAD EXAMPLE, I know I live there. Switzerland has a very homogeneous, extremely wealthy poplulation in which I gurantee no one carries a gun around and everyone knows it - so again the problem of school violence would still be totally feasible.
And also, despite all the wealth in Switzerland and the education level and the virtual absence of crime in comparison to the US, for the record italy and germany (who neighbor switzerland) who do not have guns have an even lower murder rate.