クラス in Japanese means a group of students, so 授業 is probably better.
来週の授業は、クリスマスの前の最後ですか
Is probably what I’d say while keeping your main structure
As for 最終, I don’t think there’s anything inherently wrong with it, but I don’t hear it as much as 最後. 最終的な would be used if you were modifying a noun directly. You can’t put です after it like that.