Here it is Kellogg's distribution
GMAT Distribution
UP TO 640 : 9% : 20%
650 - 690 : 25% : 28%
700 - 740 : 49% : 41%
750 - 800 : 16% : 11%
Average : 703
Chicago is not one of the toughest schools to get into. At least according to PR:
http://www.princetonreview.com/mba/research/rankings/rankingDetails.asp?TopicID=1