Huffington Post asks this question.
For me, I never think of a child from a challenging home life as needing that home life graded. I'm not sure what grading parents would do, nor how I would go about doing it as a teacher. And honestly, I've worked with very few parents, even ones that struggled to meet the needs of their child, that I couldn't find something that I thought they did well. So I think I couldn't support grading parents, as for too many parents, they are already wary to work with schools and teachers due to the parents own poor experiences in school. Worrying that they will be graded poorly will just increase their negative view of the school. While I think grades might make some parents more involved, for many, I think they will likely be turned off.
Florida Lawmaker Wants Teachers To Grade Parents