You don't. We just try to disrupt attacks before they start. Columbine High had an armed guard, the school in Beslan had an armed guard (who actually dropped one terrorist before being killed). New Life Church here in Colorado Springs was a more positive example - that armed citizen dropped the shooter before he could complete a full-scale massacre, but not before he killed two teenage girls.
Stay alert, get a concealed permit.
Plus, whenever concealed carry passes crime stats in general plummet, terrorism is, after all, just another crime in a big long list.