Be honest and upfront. Going behind her back would make her wonder if she hid this what else is she hiding. The point your making here is you are getting older and want her to see you as a young adult not a kid. But going behind her back would make her distrustful and wouldn't help you. Sit her down and talk to her. Explain your feelings but also tell her you understand her concerns. Then ask if there was a way yall could comprimise. Ask her if the both of you could settle on some guidelines. Also let her know that you would promise to only wear it at home. Explain that you would never wear it at a friends or out in public. That will suprise her and she would see that your handling the situation in a responsible manner and that your being mature. that's the best way deary.