To, [Teacher's Name], [Subject] Teacher, [School Name]
Date: [Date]
Subject: Apology for [mistake, e.g. talking in class / not submitting homework]
Respected Sir/Madam,
I am writing to sincerely apologise for [describe the mistake briefly, e.g. my behaviour during yesterday's class / failing to submit my project on time]. I realise that my action was wrong and that it [consequence, e.g. disturbed the class / showed carelessness towards my work].
I assure you that this will not happen again. I have already [corrective step, e.g. completed the pending work / spoken to the classmates I disturbed], and I request you to kindly forgive me this time.
Thank you for your patience and guidance.
Yours sincerely, [Your Name] Class [Class], Roll No. [Roll Number]