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 students,
Tom