An additional lesson which takes place only once can be created in the scheduling dialogue by the means of a special duty window or by an entry in the details window (see chapter. "Special duty"). However, you can also add already existing lessons.
How to add a teacher from a class' point of view
Set the scheduling dialogue to "Class". Now you can add another coupling line in the details window.
How to add a class from the teacher's point of view
If you want to add a class to a lesson, you simply do the same as above. Set the scheduling dialogue to "Teacher", and enter in the "Class" field the additional class, separating it by a comma.
If you set the scheduling dialogue to "Class", you cannot modify the entry in the "Class" field. However, you can switch to the teacher's view. The same applies for teachers.