Can't say that I agree ragman.
It was apparantly do the OP a lot of good, giving her closure.
As for the person getting the letter, it certainly won't hurt her.
If the teacher was hurt, she now has an apology, if she wasn't hurt, doesn't even remember the situation, or see the situation in a negative way, she now has the pleasure of knowing someone thought enough of her to make up for something.
If I got a letter like that, I would think "There's still people around who think of the feelings of others"