Simply replace the unfilled areas such as "" and "[date]" with your information.
Dear [name of teacher],
I would like to inform you of my intent to drop your course this semester. I would like to take this opportunity to let you know that my decision to drop this class did not have anything to do with you or your teaching style. In fact, I look forward to being able to participate in future classes taught by you.
The decision to drop this class at this time is a completely personal one. I do not have the time to properly devote to this class, and would prefer not to waste either your time or mine when I am not able to fully commit.
Thank you for the time that you did spend with me, and I hope to be able to attend future classes with you.