楼上写的蛮好，是标准格式，考试上可以这么写，但是日常没必要这么啰嗦。直接这么写就行。
January 5th 2010
Dear Sir,
I'm sorry to tell you that I can't go to school today, I've caught a cold last night, and I feel really bad this morning.
So I want to ask for one day's leave.
Sorry for being absent。
Best wishes
Your student
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