Like anything the reality is likely somewhere in between. We often shift workouts on the day and sometimes even in the middle of the workout. But there also is a framework or a “big plan” or where we want to go and how we want things to progress.
Like you said, there are just too many variables to consider, especially teaching and coaching teens. Up all night on the phone talking to a friend? Studying all night and not sleeping for that AP test? Not eating lunch (super common). Someone posted a pic to Instagram but didn’t tag you? (Day ruined). That said, I think having a general plan is a necessity… otherwise you are just making it up as you go and that certainly can help you be better but is hardly optimal.