It's not an excuse - it's just the way it is. If kids don't have parental encouragement or involvement at home, they won't see the value in education. I taught in a rural district in the South that epitomized this. Cultural/familial attitudes towards education matter a lot more than anything the school can do. After all, kids still spend more time with their parents and family than they do at school; those attitudes rub off.

And it's not passing the buck to the parents; we're not asking the parents to teach them, merely to be involved, take an interest in the kid's education and show them the value of an education and encourage them to do the stuff they learn outside. The school can't do it all, nor should it have to (I'd argue teachers are already doing a lot more than they were ever trained to do because parents often don't have time to do it in our current society).