Most humbly, my name is _________ (name) working as a ___________ (designation) in your reputed school.
I am writing this mail to bring to your attention that my delivery date is due on date __/__/______ (date). So, I sincerely request you to sanction me maternity leave from __/__/______ (date) to __/__/______ (date).
I hope that you will consider this as a genuine request and approve my request as soon as possible. I assure you that I will rejoin the office from the first week of _________ (mention month).
Thank you for your valuable time and consideration. I am waiting for your response.
_____________ (Contact details)