Answer:
Be respectful making your decision clear, but also emphasise your interest in drama generally.
Explanation:
This is strange to explain, but I'm answering this because I faced a similar issue when I wanted to leave the school play a few weeks ago. You should email your teacher in a professional way, as if you were writing a small letter, but explain why you can't do/don't want to do drama club. Here is the email I sent to my teacher as an example:
Dear Ms ...,
I would just like to inform you that after looking at the schedule and thinking logistically, I do not think I can commit to being part of the ........ production. This is mainly because I have an extremely busy schedule and would not be able to manage attending the rehearsals. Although I would have enjoyed being a part of this production, I believe that unfortunately, it would not work out.
Thank you for your kind understanding.
Kind regards,
....
