We recommend you to write such answers using your own thoughts. However, a sample letter has been given for your reference:
April 23, 2010
California High School
Subject: Application for leave.
I am a student of your school, studying in class 10th- B. I will be unable to attend my classes from 14th January ,2013 to
16th January, 2013 as I have to visit my sick grandmother who lives in India.
Kindly grant me leave for three days and do the needful.