Dear Mr/Ms Jiang
I was unable to come to 1st and 2nd period today.Due to the rain yesterday, I was wet and had temperature at night. The doctor adviced me to rest for several days and after my recovery, I will attend school as soon as possible.
I am sincerely sorry about the inconvenience. However, I would like to have your permission. Also, afterwards I would phone you and there's the certificate by the doctor.
Li Tie
October 21st 20xx