Dear Sir or Madam:
Dear Mr Li/Miss Li/Mrs Li,
I beg to apply for two days’ leave/a week’s leave/7 days’ leave of absence （请假） from the 7th to 8th of this week. Because I was ill, I want to return/go home to see the doctor.
To support my application, I herewith/enclose the sick leave
I should be very much obliged if you will grant me my application. As regards the lesson to be missed during my absence, I will do my best to make them up as soon as I get back from leave.