Dear Mr Chen,
I'm sorry to tell you that I can't go to school today. I've caught a cold because I swam in the Kunming Lake yesterday.
This morning I had a headache. After taking my temperature, my mother found that I had a high fever. She took me to see a doctor. The doctor advised me to stay in bed for two days.
Now I'm writing to you to ask for two days' sick leave.
Yours respectfully, Eng lishtang.com