I posted something on a similar subject earlier at:
http://130.94.21.174/gcforums/showth...275#post143275
Sorry about the bad rep. That can really be a problem for both fraternities and sororities. I have a few ideas on the subject which might be able to help you. The best thing you and your sisters can do is be at your best all of the time. Be courteous and friendly ladies to everyone. You can win your campus over one person at a time. Participate in all campus-wide and greek-wide events in order to get your name out. When you have a philanthropic or charitable event, get your campus paper to cover it. Once a week your sisters should all wear letters and have dinner on campus. The key is visibility. You aren't going to change people's opinions by staying in your house and hanging out. So go out and meet people and your rep will change.
Regarding GPA: There is a minimum GPA standard that all students must meet to be in good standing and graduate. You need to set a minimum GPA in order to be a sister in good standing, such as the campus wide average GPA. You can match up sisters who have the same majors as study partners. You can even have those with higher GPA's tutor those with lower ones. My sorority gave awards every semester to sisters with the highest GPA or the Most Improved. An incentive/reward can be that the sisters who receive these distinctions get first choice of rooms in the house. The study hours need to be enforced just like any other mandatory event. If a sister misses 3 study sessions, she must pay a fine. Personally, I hated study hours because I preferred studying alone in the library so you need to take personal idiosyncracies into account, especially if she has a solid GPA. Just a few suggestion to consider. Good luck!