Everything you could possible want to know about wait lists is discussed on the 126 pages of this thread:
http://talk.qa.collegeconfidential.com/prep-school-admissions/1300302-the-wait-list.html
No one knows what anyone’s actual chances of getting off any wait list are, but you can assume they are very slim. The best advice is to move on with alternate plans. If you should happen to get off a wait list, great, but don’t plan on it