To, The Principal, [School Name], [City]
Date: [Date]
Subject: Permission to participate in [event name]
Respected Sir/Madam,
I am [Your Name] of Class [Class], Section [Section]. I have been selected to participate in [event, e.g. the inter-school science quiz] to be held at [venue] on [date].
I request you to kindly grant me permission to participate and to treat my absence from classes on that day as on-duty. I will make up for the missed lessons.
Thank you for encouraging students in such activities.
Yours obediently, [Your Name] Class [Class], Roll No. [Roll Number]