How can one plan a class?
If you see a path in front of you and you decide to follow it, planning how to do this is easy. ;-)
I indeed plan for weeks and even periods of months or half-years. Since I began to teach I always had ideas of what I wanted/want to teach and what I wanted/want to develop in the long run.
....you never know who is going show up.
I am lucky because there are certain students who show up in every class I teach. They build a core of people who expect me to teach the specific way I do. Together we proceed. There is allmost noone showing up arbitrary.
A plan would ruin the moment presented by the circumstances.
When I teach others, I try to mould, to form, to shape the circumstances. This to me is one of the most important points, what teaching is about.
When I have something to pass on to a student, there is also a responsibility on my side to make it possible, he can get it, take it.
On the other hand you are right: It is important in life - not only when practicing - to flow with it. And not to disturb or ruin it's moments by going against them.
But this is not a question of black or white I think: I always plan my practice. But I never stick to my plan when life gifts me with moments, circumstances I didn't foresee. Those things are enrichening when one is able to embrace them.
I love both ways.
Saying: "Wow, we finally made it!!!" When we achieve something, we aimed for. Sometimes over months.
And also saying: "Aha?!? This was completely different from what I thought we would do today. But was interesting, wasn't it?"