How do you say sorry to your teacher?

Apology letter is to say sorry for the mistake you have committed,so while writing an apology letter,your language should be polite.Be generous enough to say sorry.

You write in the letter that you're very sorry to start off with ,then you promise what ever happened will NEVER happen again.

Be a bit formal in Words because you are writing a letter to a teacher..

Refer the link for sample apology letter..