It was on April 20th, 1999 that the regrettable tragedy, that ended in the murder of 13 students, happened at Columbine High School in Littleton, Colorado. The comic that would have been Hellblazer #141 was still going through the publication process and was eventually pulled from the schedule due to the heightened sensitivity regarding such matters. DC considered making changes to the story but writer Warren Ellis refused them and soon quit the series.
You can now read this story with art by Phil Jimenez, in the Vertigo Resurrected #1. This anthology includes that story and more from the likes of Brian Azzarello, Grant Morrison, Garth Ennis, Frank Quitely, Jim Lee and Tim Sale. It's pricey at $7.99 but with the wealth of talent in the book, one could suppose they feel justified you'll get your monies worth. Go find it at your Local Comic Shop!