Whilst they only cover a small fraction of US homicide, the odd thing about school shootings in particular is who is doing them. In many cases these people come from families with social backgrounds that, whilst not always affluent, are not necessarily 'poor' either.
There IS, I think, a deeper-seated issue of 'expectation versus non-importance', America has the same spectrum of achievement as any other country, but American culture does tend to embrace the idea of 'if you are not exceptional, you are just background'.
I think the reason you don't see many, if any, school- or mass-shootings among the very low-income groups is because they've grown up more or less knowing that the chances are they will end up in a medium to low income job, whereas middle-class American teens, I think, find themselves at a point in their lives where their self-image is more important than ever (as it is with every teenager, everywhere) and realizing that they stand a good chance of just being 'average' at best.
Elliot Rodger is an extreme example of this, someone who was convinced he was, because of his social position, entitled to have everything work the way he wanted it to and desperately wanted that 'Exceptionalism' that is so treasured in capitalistic mentality. As it was, he found a way which, in his mind will always make people remember who he was.
That's part of the meme problem I think, without belittling school shootings, they are the ultimate tantrum, and because school shootings tend to get massive amounts of media coverage unstable kids realize that it WILL get you noticed.