When you drop a student from your class your student loses access to their course work, and your used seat count goes down.
Dropping a student
Click My Classes
Click the class name
Click View Class & Roster
Click Drop next to the student's name in the roster under the Students section
To undrop a student, click Restore next to the student's name in the Dropped Students section.
Accessing the Work of a Dropped Student
If access to past work is needed, it is only available in the "Grade by Student" view.
Click on the student's name in the "Dropped Student" list.
Click on the activity you need to view.
This will open that assignment in the grade by student view.