In: Computer Science
                                                                                            <Place>,
                                                                                                                        <date>.
To 
The Principal,
<Institution name>,
<place of institution>.
Respected Sir,
                       sub: re: leave approval for 1 day.
        I'm <name> pursuing <Std> with the no <reg no> requesting you to permit me leave for one day, as I've to work as a tourist guide on a part-time basis, due to the heavy work for me for the earnings.
                                          Thanking you,
                                                                                                            yours faithfully,
                                                                                                            <name>.