Where are the statistics on parental literacy per school or school district, correlated with the literacy of the students?

A significant portion of the US population is illiterate - 15-20% or more.
School performance in terms of literacy (and arithmetic) correlates with the socio-economic status of the school district.
If there is a high degree of illiteracy in the parents of the under-performing school children, then why aren't we trying to educate these parents. The chances are that these same parents are significantly under-employed and therefore available for literacy tutoring.
With increased parental literacy, the expectations for the kids will be raised by the parents and fewer kids will be left behind.