9 GCSES is completely fine. I would suggest talking directly to your parents and telling them that it is reallly influencing your other gcses (even if it's not, telling them might make them more likely to let you). As long as your school is ok with it, I don't think it should be a problem. You could also possibly tell them that universities sometimes also look at gcses and the 2 would look bad? Also say that you will improve your grade in subject x if you drop, and try to do that so they know they can trust you. How are your grades in the other subjects?