The best way to convince your parents to let you go to a boy/girl party is to be honest, truthful and give them hard facts about the party. If the party is legit with supervision then give them the phone numbers of the adults at the party. Let your parents speak with other parents who are letting their kids go to the party. Let them know they can reach you on your cell phone at anytime and/or you will keep in touch with them by calling them every hour.
Parents feel better if they know they can trust their kids and are able to keep in contact with them.
Well all you can really do is ask. You can try explaining that you are smart and won't do anything stupid. If there is going to be adult supervision at the party maybe you can get your parents to talk to this other adult and they can put some of your parents worries to rest.