How can i get my gma to give me some of my money to get school clothes?

I have money going into the bank for things i need (clothes, food, fines lol) and i want to get 400 bucks out to buy new school clothes. My gma probably believes that i dont need more clothes but its my first year of high school and want new school clothes. I'm afriad she'll say no but why would she when its my money for what im asking for it for. How can i convince her. i didnt ask yet but i am later. Should i tell her ill get rid of asa many clothes that i have now as im gonna get? what should i say/way should i handle this in