Be it hospital, courthouse, school, whatever, an armed guard or two standing at a metal detector at the entrance will not stop, or even slow down, a group of armed badguys intent on taking the facility over. The guards will simply be the first to fall, and any innocents nearby will become hostages, to prevent any aggressive response by other guards within the facility. In my opinion it is just about impossible to keep determined badguys out of any public building. Therefore a plan must be in effect to prevent the BG's from taking over the whole building and everyone in it, limiting access to rooms and the people within. If the responding SWAT teams know that the BG's (and the original hostages) are in the hallways, their job will be a little easier. Tall order...