thanks. i will call you at 1:30p.m. or you can call me at any time.
one possible version:
june 8, XX
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