AP teachers can drop students from their class sections if you’ve left that functionality enabled. See How do I complete initial setup in AP Registration and Ordering?
If the student’s Order Exam? status is Yes and the exam order has already been submitted, the student is automatically moved to an exam only section named Dropped Students for the course after the teacher drops them from their class section.
This section functions like any other exam only section but is created automatically by the system when a student is dropped. If the student no longer intends to take the AP Exam, you’ll need to remove them from your exam order and submit that change if your initial exam order has already been submitted. See How do I submit a change to my initial order?