Exceptions are separate from enrolment. The enrolment for a class can be managed through Course Enrolment by Class. In this case, the Class List Exceptions available through the Timetable allow you to specify a student or teacher who will not be in that class for a particular period. Exceptions are likely to be a rare case, but it’s useful in scenarios where you know ahead of time that a particular person needs to be excluded from the class list for a certain period (eg: a class runs Mon, Wed, Fri, but a certain student doesn’t attend on Fridays).