My parents think my girlfriend is a bad influence and will no longer leave us unsupervised, what should i do?

Let them get to know this girl and see that she is really a great person. Going behind their backs or being rude to them will get you no where; actually, it will take you a few steps back and confirm their fears. They are your parents, and as unreasonable as they may be, they just want to protect you. Let them get to know her, and in the meantime, enjoy her company with your parents. You also have the opportunity to be "girlfriend-y" with her during sleep overs. After a while, your parents will get sick of having to accompany you everywhere and will realize that your friend is a good person. Good luck!