Most respectfully,
I am a student of ______ (class) of your reputed School and my roll number is _________ (mention roll number).
My name is ________ (name) and I am suffering from fever due to which will not be able to attend classes for today i.e. _________ (date/ day). Moreover, I ensure you that this will not be affecting my performance and I will be working hard to complete the pending work for the said day.
I expect your kind approval for taking leave for the said date. I shall be obliged.
Thanking you,
_________ (Name),
_________ (Designation)