Dear Mr./Ms. XX,
I am sorry to tell you that I am not able to go to class today,because I have to see the doctor.I had a bath in cold water last night and my head is aching badly ever since I woke up this morning.I probably got a cold due to the stupid cold bath.I hope the class without me today will be interesting as usual.:)
a most lovely but unluckily student of yours
June 2,2010
这样写既能说明原因又不乏幽默,老师应该会批准的