Dear Mr/Mrs/Ms XXX（teacher）:
I am writing this letter to ask for your pardon for my absence in your class.
As CET-6 is around the corner,I realized how much I am lagged behind for my preparation schedule. The only choice I have is to borrow some time from my classes for this upcoming exam which is so important for I am terribly sorry that I cannot make to your class, and I would be grateful for your understanding.
Thank you for your understanding and I do appreciate ir very much. I will make every effort in making up what I may miss from your lesson.
Yours
XXX