When you withdraw them, make sure they do not have a "Next Year's School" set on the biographical profile page.
Article ID: 153
Last updated: 26 Aug, 2009
Attendance -> FAQ -> If a student is withdrawn, how do I make sure they don't get rolled over at the end of the year, or show up on next year's list of incoming students?
http://www.imgsoftware.com/kb/entry/153/