Answer

Suppose if we want to write an application for a sick leave, it can be framed as below:

Format:

To,
The Principal,
______________(name of school/institute in place of dashes),
___________(name of place where the school or institute is), ___________(name of District),
_____________(main city, like- New Delhi etc "don't use this line if not applicable).

Respected Sir, (use Ma'am if the Principal is female)

              I beg to say that that I am suffering from ________________(headache/high fever/jaundice/stomach ache or whatever disease/problem you're suffering from) since ___________(mention the time, like- last night or two days etc). So, I am unable to attend the school/class(in case of institute) today(mention the date also)/ from (Date:dd/mm/yyyy) to (Date:dd/mm/yyyy).

Kindly grant me leave for ________(mention days you're going to be on leave, like- one day, two days 10 days etc).

Thanking you, 
Yours faithfully,
_____________(your full name),
______________(class with section which are in).

Dated: dd/mm/yyyy (date of submission of the application)

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Note:

1. The words in 'italics' are used here just for the purpose of guidance/help, so please do not write it into your application.
2. The starter sentence of the main content i.e., 'I beg to say that' can also be written as 'It is stated that'. Well, it is all up to your ease and choice of selection.