I work with mentally ill patients, and I was an active SWAT officer when Columbine happened. It changed how we did everything.
After Columbine, we got our floor-plans on ALL of our local schools, and spent hours and hours during the nights assaulting those locations, and gaming-out active shooter scenarios. We had other officers play the OPFOR, and hunted them through the hallways. What we discovered was that as fast as we were, we weren't fast enough. By the time a police response arrives at a school, the gunman can have already killed several dozen (as happened at Virginia Tech).
The answer to a "man with a gun" is another man with a gun, and the School Resource Officer is critical against a homicidal maniac. The faster you can get that man on-scene and putting rounds on-target, the better.
And our mental health system is badly broken. Look into the eyes of Lanza, Holmes, Loughner... it doesn't take a board-certified psychiatrist to tell you they've lost touch with reality. Unfortunately, there are very few resources out there to address people like that. Until that changes, people like that (though they throw up red flags to every person who knows them) are going to continue to fall through the cracks.