Following is a letter written by Kailash to his class teacher, requesting leave for a marriage. You can use the same format by simply replacing the name with yours and adding your school details in respective places
The Class teacher,
My sister’s marriage is planned to be held on (Date 1). Hence I will not be able to come to school for 3 days next week.
Kindly accept my request and grant me leave for 3 days (Date 1, Date 2 and Date 3).