Dear Mr./Ms. Bossname,
Thank you for the opportunity to serve as a tutor for the (Term) to (Term) school period. However, I must resign my position, effective at the end of the term.
It has been a pleasure and a privilege to help teach, support, and encourage students in (program/subject), and the opportunity has afforded me much in the way of experience and knowledge. After this term, I hope to use that knowledge for (next career step).
Thank you again for the services you provide, and for allowing me to be a part of your program.