There really is no average. Everyone is different. I’m a 28f or a 32d(using the 4 method) and personally I think I’m average ngl. I know people who have huge boobs and people who are as flat as a board. I’m 14 btw but this still goes for 13yos.
Everyone has different bodies that come in all shapes and sizes - and grow at different times! There is no 'normal' when it comes to boobs, because different womxn start puberty at different times and therefore grow boobs at different times. One girl might grow boobs at age 11, and another at age 15/16. So, just know that once you finish school, everybody will be fully grown and the differences won't be as big. While you might feel different now, you won't by then.
An average 13 year old’s bra usually ranges between 32 AA to 34 B but that is just average there is nothing wrong with having smaller or bigger than those sizes. Hope this helps
The only answer to this is 'how long is a piece of string'. Some girls have massive breasts some could be mistaken for a boy, size wise. It makes no difference though so dont worry about it.