A student who elects to withdraw from The University by dropping all classes after having paid registration fees must initiate such a procedure by contacting the Dean of Student's Office. A withdrawal may not be initiated after the last day of classes of any semester, and must be completed before the beginning of the final examination period.
Consult the Schedule of Classes for detailed instructions and deadlines.
Medical withdrawal is initiated from the Student Health Service.
Adequate medical documentation must be supplied by the student.

The official policy can be viewed at http://catalog.arizona.edu/2008-09/policies/leavingu.htm#withdrawal.