A class can be defined as a group of users (teacher & students) that follow a weekly, recurring lesson schedule
A lesson can be defined as a single, non-recurring event that takes place on a specific date and time. In this way, the student list for a lesson can be different from the student list in a class, as a lesson would frequently include make-up and trial students.
The simplest way to think of a class is to think of it as a cup, and students that are added into the class are like marbles in a cup. The class is used to store the list of regular students in order to automatically update the student attendance list in all lessons.