Sometimes the start of the lesson is driven by the need of whoever turns up. Sometimes it is driven by what happened previously. Sometimes it is driven by what I am currently interested in.
In all cases, exactly who turns up will potentially change what is taught.
Equally, there may be a starting theme, but what that leads to depends on who is in the class and what they do. On the occasions where I have tried repeating a class that worked well (in terms of sequence of techniques etc), the lack of spontaneity and reaction to what is happening tends to make it turn out less well...
That said, I have a duty to prepare people for exams and to cover areas of weakness, thus can't just ignore totally areas of the syllabus.