At my school, girls wore dresses like that on Meet the Greeks day. At pref - most of us wore more dressed up clothes.
Greek Life didn't send you a rush packet with what to wear and what not? Usually they do. Ours went like this (and I think it is similar at most schools):
Day 1 (Meet The Greeks): Sun Dress
Day 2 (Philanthropy): Casual. Some people wore Capris, I wore shorts and a cute sleeveless shirt
Day 3 (Skit): Semi-Dressed up. I wore a sleevless fitted shirt with a grayish black skirt from the limited
Day 4 (Pref): Little dressier than church clothes. I wore a strapless black dress with different flower prints and the bottom angled down to one side:
Anyways - I was stumped as to what to wear until they sent me a booklet explaining all that stuff. Also remember to pack way more than you actually need. It sucks getting there and realizing you do not have enough clothes for activities outside of parties.