Should I wear a bra? I'm almost 13?

I have some but i'm like a 28AA...really flat, but like in the corridor in school if i get banged into or whatever it really hurts, 'cause i'm really tender, and when people find out i'm not wearing one their like o_O. And all my friends are mive, and wear them, I really don't mind them,plus when I wear thin tops you can see my you know whats sticking out! Oh btw im going to be 13 in February and i'm in second year. Should I?