Dear Sir,
I'm sorry to tell you that I can't go to school today, I've caught a cold because I went back home in the rain without umbrella yesterday ,so that this morning I had a headache. After taking my temperature, my mother found that I had a high fever, immediately she took me to see a doctor, who advised me to stay in bed for one days. Now I am writing to you to ask for one day's sick leave.
With best wishes
your student
Tom