The situation is a little...unique. I must honestly admit I am disappointed in your teacher for showing such a lack of judgment in the matter (by having a sexual relationship with you to begin with). He is putting virtually his entire life on the line for this fling with you. I would like to reassure you that the guy is clean but if he is doing this with you, who knows who else he has done this with. What other kind of bizarre fetishes does he have? You have a tough choice to make. If you are truly worried and want to get tested, you will have to explain the situation to your parents. My opinion: better safe than sorry.