That doesn't justify shooting the kid, but the school district officials who allowed his harrassing behavior should bear most of the responsibility.
It seems unlikely that this effeminate boy was doing much bullying. As no doubt he was being harassed much more than he was harassing.
Dressing that way was a class disruption as well as subjecting himself to forseeable ridicule. Do homosexuals honestly expect this to not push the buttons of certain individuals and create a potential atmosphere of harassment and violence?