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 me. 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