I have had a few lessons where I have been out of town for several days, thus reducing my practice opportunities. Usually I just tell the teacher when I walk in, and suggest she not expect too much.
Oddly, it is funny, but some weeks I do not play very well, even though I have practiced quite a lot. Other lessons where I have practiced no more than any other week, my teacher "passes" me on the pieces I have ready, and may even praise me (rare).
On one occasion, I thought the lesson went badly. However, at the end of the lesson my teacher said, "I think you did wonderfully this week."
The reason -- the piece I was playing was very difficult to read from the music -- numerous accidentals, double flats, complex bass chords as many as 4 lines down from the staff, and the like.
Samuel Johnson once said about a woman preacher, "It is not that she preaches so well, but that she can preach at all." (He had low expectations from the distaff side, it appears.)
So I guess her point is, not that I played it that well, but that I could play it at all.