Except VERSUS excluding
1 sounds like you're not actually having this lesson. "Except" seems to refer to "had", and so it means that you "have not had" this lesson, and furthermore, it appears to say that you are not going to have it. Hence in the context of this sentence, it's close to nonsense (How can you not have a lesson you are already by implication having?).
On the other hand, the "excluding" in 2 refers to the count of the number of lessons you've had.
It's tantalisingly subtle.