To, The Principal, [School Name], [City]
Date: [Date]
Subject: Application for sick leave
Respected Sir/Madam,
I am a student of Class [Class], Section [Section]. I am suffering from [illness, e.g. a high fever] and my doctor has advised me to take rest for [number] days. I am therefore unable to attend school from [start date] to [end date].
I kindly request you to grant me leave for these days. I will complete all the missed work on my return.
Thank you for your kind consideration.
Yours obediently, [Your Name] Class [Class], Roll No. [Roll Number]