Not every sorority is different, but NPC handles thing a lot differently than other governing bodies, locals, etc. There are rules on everything, and the rules are strict.
To the OP, don't worry about your grades suffering during your "pledging." While you will have commitments to your house as a NM, there will be ample time to do schoolwork. Moreover, you will most likely have manadatory study hours to ensure that your grades don't slack. Most houses are adamant about having stellar GPAs.