I was wrong... "Zero Hour" is a series that runs on Discovery, not History... so, for anyone who was interested in seeing the one on Columbine, watch your listings for a rerun.
http://www.discoverychannel.co.uk/zerohour/series1/columbine_high_school_shooting/index.shtml
There seem to be some links here where you can watch it online, but I haven't checked any of 'em out:
http://video.google.com/videosearch?hl=en&q=zero+hour+columbine&oe=UTF-8&um=1&ie=UTF-8&sa=N&tab=wv&ei=Cl7vSZPWNseDtgfv-cnRDw&oi=property_suggestions&resnum=0&ct=property-revision&cd=1#
Looks like "Zero hour columbine 1" and "2" and "3" would cover it.