The episode is called "Jimmy" and aired in 2002. It's hard to believe this was on Kids WB. Like even without the blood, this is some heavy stuff for a kid to see and process.
With how common school shootings are now, it makes me wonder if cartoons should tackle heavier subjects like this more directly.
Dwyane McDuffie and Alan Burnett won the 2003 Humanitas Prize for this episode.