[Hartofsec]

I'd be kinda surprised if you see a lot of girls wearing pearls with their t-shirts -- IMO a long casual necklace (as suggested up thread) would be better. You don't want to look like you are trying too hard to achieve a stereotypical sorority look, and besides, perspiration is really terrible for pearls.

Of course I don't know what is customary on your campus, but you might look through photos on the websites/facebook of chapters located on your campus.

ETA: If the front design of your shirt is really busy, or if you will be wearing a hanging name tag, it might not be a good idea to wear a long necklace. If you wear a necklace, perhaps something shorter/less obtrusive would be a better choice under ^these circumstances.