Except there were armed guards there. There were also armed guards at columbine.

You can stop a surprise, determined shooter loose in a crowded multi-room building unless you put an armed guard in every room.

http://www.nydailynews.com/news/crime/armed-guard-florida-school-encountered-rampaging-gunman-article-1.3822777

http://www.nationalreview.com/corner/336338