If they are staring, you can always say, "what are YOU looking at?" Or make funny faces at them til they snap out of it. Or ask them if there's something wrong, cause you noticed they were staring.
If they are verbally hitting on you (and you want them to stop) talk alot about your wonderful boyfriend. If you don't have a wonderful boyfriend, you can make one up, who COULD own a beachfront villa and be very possessive.
If it's as bad as groping, option one is to Loudly Object - shriek "get your hands off of me, you pervert!", option two is to complain to the instructor. That's assuming they are senior to you and you can't yet beat them up (option three).
I'm guessing you are relatively new and/or relatively young.