By Brother is indeed right: the "Livingston" ammendment to the Higher Education Act can be found at the link below. It precludes schools that accpet federal funding from anyh infringements on association rights. Practically, it means that your chapter would have to be sanctioned for VIOLATING the policy before seeking relief - however, I have found that raising the issue with university General Counsel to be pretty effective.
The ladies make good points; however, we cerainly have more than enough examples of chapters that take first semester freshmen and their grades do not suffer, but are indedd above the rest of their classmates on average.
http://www.ed.gov/legislation/HEA/sec101B.html
Brad