I have no idea about southern rush dress guidelines, but strapless/church attire things are tricky. Personally, I think people should have their shoulders/thighs covered in Church, but many people do not feel the same way and I see lots of bare shoulders and short dresses/skirts in Church. This probably depends on the region, and I'd call the school and ask to clarify on "church attire"
http://www.jenclothing.com/ where I like to buy modest dresses (I have no affiliation with this site and will take this part down if need be!)