Dear Mr. Wang,
I am very sorry that I shall be unable to come to your classes today owing to a bad cold. I enclose a certificate from the doctor who is attending me. He said it will be two or three days before I shall be able to resume my school.
Dear Mr. Smith,
On account of an attack of the flu with a severe headache, I shall be unable to attend classes today, for which I submit a certificate from the doctor. I shall be much obliged if I can get your permission.
Dear Mr/Ms XXX;
Today I'm writing to you to ask for a five-day leave, for I've got cold last night with carelessness.
This morning my mother took me to see the doctor, who told me to stay in bed for some days. So I am very sorry to be absent from school, especially your interesting lessons. I'll be sure to make up for the missed lessons after I recover from the illness.
Dear Miss Gao,
I'm sorry I can't go to school today. I helped the farmers pick apples with my classmates on the farm yesterday. Unluckily, I fell off the ladder and hurt my leg, but I wasn''''t badly hurt. The doctor asked me to stay in bed and have a good rest. So I ask for leave for two days.