My friend keeps on ... ???

I am 13 year old girl and I am mid height and very skinny almost anorexic but not quite ( luckily ) and I have 30 a boobs and my friends have like 32c s or more and they take the piss and really make me feel self conscious and when we shop thy take tge Micky alot like when I was looking t the bras ge said oh I don't think ou can get them small enough here go over to the kiddy section and it made me feel like I wanna cry I also don't understand how they can be mean cuz most my friends are quite fat no offence to them but I don't say oh there aren't clothes in a big enough size I keep my comments to myself I have tried telling them to stopbt they don't I really get upset xx thx