Dear Sir,
I'm sorry to tell you that I can't go to school today, I've caught a cold because I swam in the school's swimming pool yesterday.
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 2 days. Now I am writing to you to ask for 2 days' sick leave.
With best wishes
Your student,
Tom