All the girls in my class say that I’m immature and they all have bigger boobs and i really feel insecure about it

Okay so this is totally normal, and no one should make you feel immature about your body because literally everyone develops at their own speed.

Like, some girls start getting boobs really early, and some start way later, and that's just how it is. It's not a race or anything, even though it can totally feel that way when you look around your class and everyone seems to be doing something else. Also, boobs come in all kinds of shapes and sizes, and they keep changing for years, even after you get your period.

Your body is just doing its own thing on its own timeline, and that's okay! It definitely doesn't mean you're immature. Puberty has a ton of steps, and they don't all happen at once for everyone. You're just fine exactly as you are.