Parent involvement increases academic performance
This probably belongs in the "Well, duh" file. A study published in the Journal of Human Resources indicates that parent involvement with schoolwork has a dramatic impact on performance - schools would have to spend $1000 per student to achieve equivalent effects. Parents were more interested in their daughters' work than their sons', possibly because it's easier to discuss academics with girls.
The researchers also reported that parent involvement decreases when schools increase spending, resulting in a maintenance of the status quo.