While I'm sure your premise is entirely correct, blaming parenting is essentially punting on the issue. It might be entirely accurate, but it doesn't help any, because it is one thing that can't be changed by the teacher or school administrator.
This idea that a teacher can stand up in front of the class and say something so inspiring that it magically makes the goof-offs and the slackers want to be A+ students is great for a Hollywood movie, but completely unrealistic. On a 1-on-1 basis, sure that's probably possible if you're really good, and really can take the time. But if teachers had the power to inspire and move those troublemakers en masse with the power of their words without taking half their class time, then they wouldn't be teachers, they'd be world-class salespeople or winning politicians.
When a parent can understand at what level their child is at in their educational path, then they can make an impact. Meaning, if my child has problems just sitting down and absorbing information, they can go out and find answers.
One large part of why the educational system is so bad right now, is that parents are not taking responsibility for their child's education. They blame it on others.
I'm not saying that all children should mindlessly obey whatever they're told by authority figures, but I think we can all get together on "shut up while the teacher is talking"
I mean, maybe that really is it -- we have truancy laws where a parent can get in actual legal trouble if their kid doesn't go to school, perhaps there should be laws where a parent can actually get into trouble if their kid is a disruptive student. I have a hard time seeing such a system actually working, "can't legislate morality" and other platitudes, but it's a thought, I suppose.