The teacher is also able to remove students from the classroom.

When you click on the Delete a student or teacher link, you'll see this:

To delete a student or teacher from your classroom, first click on his name in the list. Next, click in both checkboxes, then click on the Click Here button.
If the deleted user never posted any messages to the classroom, his account is completely removed from the system. However, if the student or teacher did post one or more messages, those messages will still be available in the classroom.
You can restore a deleted student's or teacher's account by adding the person to the classroom using either the "Add a student" or the "Add a teacher" options in the Configuration menu. The person's account will be completely restored.