I wasn’t there, so I wouldn’t know. You have to admit he is right though. Also it depends on where the police officers were at the time of the shooting.
Modern high schools tend to be big campuses so it's entirely possible that the police officers were at the other end of the grounds from where the shooting began. But that's going to be true of any security. Unless you have armed guards on every corridor or every classroom then there is no way to say that armed security could prevent, or even discourage, something like this from happening.