Massacre Motivation

So, I have been thinking over a potential plot idea- a school schooting on a massive scale. Basically, a gang gets mad(for some reason), and decides that about twenty members or so will attack a local school, accompanied with some students there. They seal off the school from the police and start killing kids randomly. TO survive, some students start banding together to kill the others before they kill them. The problem is, though, the motivation; why would a gang want to attack a school?

The only two reasons I could think of are For the Evulz and..........actually, no that's about it.

It's just such an unambiguously evil act that there's really no reason to deliberately want to do this unless it's just for grins and chuckles.

What if a rival gang leader is at that school, having established himself as dangerous enough to warrant drastic action to take him down?

^^ The gang leader is a little crazy, so if all else fails, I could do that.

^I was actually going to have the gang leaders more on the Adult side, with some recruits from the school. Its supposed to create discord when the gang leaders force the teens to kill their friends that they see everyday, provoking many Face heel Turns.

Here's one idea. The raid starts out as a specific mission to humiliate or kill someone in a rival gang (or a relative of same). Someone in the rival gang is in the school, sees what's happening, and calls for help. Things escalate, some by accident, some by bravado, some by retaliation.

That sounds good, actually. How much manpower would a gang actually need to overpower the local Policeforce and barricade the school?

They don't need to overpower the local police force to barricade the school, and they don't need to barricade the school if they can overpower the local police force.