This is truly a tragedy for many of these kids and their families. Two of my colleagues went to VT and are visibly affected by this event these last two days. Just overall a sad statement on mankind.
In regards to how the attacker was allowed to carry on, the reality is that these are a bunch of kids who have most likely have no training in weapons or self-defense techniques aimed at disarming an attacker. This isn't in their menu of skills.
They are in an environment that most would consider "safe". IMO, these kids didn't react or did but failed to stop the shooter because they most likely didn't have the skills and were experiencing a tremendous amount of fear and shock.
The media can quaterback this event until their heads pop off. If they want to find things that were handled positively, they will. If they want to find problems with the way it was handled, they will. In America, we live with a reasonable expectation of "safety". However, nothing is finite. We have to understand that we are open to acts of violence, acts of terrorism, etc. It has become part of our reality now, unfortunately. I wish it hadn't, but it's there. We could fence up, barricade, arm, xray and close off every building in this country, but the truth is that violence is an intrusive, invasive element that can always find a way. The media can pick apart this event, but the reality is you can't be prepared for every possible scenario.
I guess we shall see how this plays out.