Implements a mechanism we call 'sessions' (not the same as a user's session, $_SESSION or something related). In our case, Sessions are entities representing "WHEN and HOW a lesson happens in a specific course". When a class is created, we identify it's attached course. Identified it's course, we loop through the course's modules and then through the modules' classes. For every class contained in each module, we create a session. Sessions have a specific code, date/time and type (face/online). They can be rearranged through a tabledrag UI also implemented by this module.