Fantasizing about your teacher will have no negative effect on him or her. It would be a different matter were you to act on those fantasies. I'm not sure how old you are, but, if you're a minor, it would be illegal for your teacher to engage in sexual activity with you. Not to mention all of the responsibilities and potential consequences you would face deciding to have sex, period.

Some of the most exciting fantasies fall within the realm of the forbidden, so enjoy your "forbidden fruit."