Hello, I am here to help you with this question. I am not sure if you are able to copy and paste, but here it is!
Dear [insert teacher's name],
I am [your first and last name] from your [insert class period]. I have emailed you to let you know that I am not very interested in after school choir, but thank you for the offer. Have a wonderful day!
Sincerely, [your name].
An email to your teacher doesn't always have to be necessarily long. Just follow that format every time you're writing an email to your teacher. Be sure to proofread before sending, though, just to make sure your grammar is clear and everything is okay.
I hope this helped you!! c: