Write an application to your principal, requesting for leave to attend your brother's marriage?
Here is a sample leave application for your reference:
1st October, 2013
Subject: Application for leave
This is to bring to your kind notice that I am studying in Class VIII of your school. My brother's marriage has been scheduled to be held on the 22nd of this month. I will not be able to attend school for 5 days as there would be a lot of guests at home and I will have to help my parents in the preparations and looking after the guests.
Kindly grant me leave from 20th October to 24th October. I assure you that I'll join back on 25th October positively.
Kindly do the needful and oblige.