If you're still young enough to need your parents' permission, then you're too young to go against their wishes. You live in their house, you eat their food, you spend their money, you use the things they buy you. When you're financially independent, you can decide how and if you want to rebel. Until then, have a little respect for your parents and their feelings.
If they said no, that means no. Listen to your parents, they wouldn't want you to walk around looking stupid or be seen with a child like that. They are trying to keep people from picking on you. If you were to go ahead and do it, your in for some big trouble.
When my daughter was13 she shaved off half her hair. I quit my job so she wouldn't do any more stuff like that. I made her wets a football helmet at home every time I came home she had to put it on cuz the sight of her with only half her hair made me cry. Tell your parents you'll wear a football helmet at home if they will let you. They might go for that! It will save them a lot of tears!