I agree, your sensei should be observant, and handle the problem. I am also a teenager in the martial arts, and I haven't had a problem with others. But my sensei is strict in that if anyone is disrespectful to another person, then they will be kicked out of class. I think you should talk to your teacher. And if worst comes to worse, and he/she can't do anything, then just ignore the woman. Nothing must waver your focus in training. I hope you are able to stop her hateful conduct though. good luck, and never let anything bring you down, and if you do, let it be a kick or punch, not mere words.