Well I would assume that the sorority girls would be pretty trendy, and also understand a college student's budget. I think a blazer and turtleneck might be kind of conservative for the average college student. What you have sounds fine. I'm still in high school, but when I'm told to dress "business" for an event with my peers, I normally pair a skirt or dress with a cardigan or blazer, nice jewelry, and heels. If I am meeting with older people, I'll dress more conservatively, or wear a suit. I'll bet most girls will dress like you, or wear skirts and cardigans. I'd wear what you have, and hope they accept you for you!