this is going to sound dumb, because youre in high school but ill bet this idiot likes you. he is attempting to hone in on anything that makes you feel uncomfortable, dont let his ignorance get you down. I think youre looking for belittling comebacks to shut him down. I was never good at comebacks on the spot either, now I'd have to be there to hear what he said..... omg. anyway, you could always say something about his lack of knowledge. ahahaha. you know whats so funny, the same dudes who bully you now will be asking you on dates later on, they did it to me. the best comeback is to not date their weird ***.
the important thing is you know you are not ugly, so who cares what a loud mouth, attention seeker tries to say to embarass you. he also sounds like a troll, and of course "dont feed the troll", meaning ignore his attention seeking behavior.
YOU COULD ALSO try to ask him whats his problem.