At the end of each day, it all comes down to this:
1. What did you want each student to know/be able to do?
2. How do you know that each student knows it/can do it?
Ensemble directors are usually pretty clear on #1 (particularly the "doing"). But #2 is about being a teacher, not just a director. Assessment in simplest terms is finding out whether or not the teacher was understood by each student. The concept was not taught simply because it came out of your mouth, the concept was taught because the student understood you.
It is a perfectly reasonable expectation that a teacher (a) knows what each student does or does not understand and (b) modifies instruction accordingly. This is also the most challenging aspect of what great teaching demands.
And by the way, this expectation has always been the truth of the matter for ensemble directors...this is not news. It's just that these days we are being asked to "show our work" like our other colleagues in the building. This is a good thing.