Looking at a lot of the background characters during the school's Halloween party, I noticed that pretty much everyone's costume is based on an existing DC character or a well-known pop culture character.
But while looking around I noticed these two costumes in particular:
http://img834.imageshack.us/img834/8461/wghq.png
Wonder Girl and Harley Quinn - Does anything else hope that this might be comfirmations for their future appearances? 8D
(On an unrelated note, I also saw one girl in the background wearing Miss Martian's uniform...o3o)