have you voiced your views to your parents? be mature, but calmly explain your views. if they still say no you have to listen if you're under age. i wouldn't advocate going through a religious ceremony you do not agree with, but it isn't legally binding, and i believe is pointless if you do not have that connection to God. i was forced into confirmation as a kid, i didn't understand it, i just did it because everyone else was and therefore i had to. i believe if you do not feel the spiritual connection any religious ceremony will not matter because it did not matter to you. i would like to believe however if you explain why you feel Wicca is right for you, your parents will understand. [but then again i grew up in a liberal household]
you could also try making a few personal changes, like call on the Horned God, don't call out 'Horned God' in church mind you, but you can visualize and reflect on your God during the confirmation.