I've been in this situation (manga-comment) a few times, and I have to say the same thing every time. I first noticed my classmates (plural) wearing a bra in 5th grade (11yo). And in 6th grade at least 2 of them were definitely bigger than this girl. Notice other girls here aren't quite as developed.
So personally I don't have an issue with 2/3 girls out of 15 (if we go for a 50/50 ratio) being somehow developed. What does bother me a bit is that they don't LOOK 12. Body proportions, face shape, etc. The boobs are just an extra.