To, The Principal, [School/College Name], [City]
Date: [Date]
Subject: Application for one day's leave
Respected Sir/Madam,
I am a student of Class [Class], Section [Section]. I have an urgent piece of work at home on [date], because of which I will not be able to attend school on that day.
Kindly grant me leave for one day. I will complete any classwork or homework I miss.
Thank you.
Yours obediently, [Your Name] Class [Class], Roll No. [Roll Number]