"Body shaming" is defined as the action or practice of humiliating someone by making mocking or critical comments about their body shape or size. Look it up. It doesn't require photos. And it doesn't matter if she said it publicly or privately. A private conversation criticizing her regarding her body would be humiliating as well.

In what universe does any teacher's job include telling a student to shop in the plus size department or that she's going to get in more trouble because of her large bust? You really think such a conversation, even with no audience, is ever appropriate coming from a teacher? This is a parent's issue to address, not a teacher's.  If she violated dress code, write a note and send her to the office. Any further criticism of appearance is uncalled for. 

And for your pious "I won't assume she said it in front of the class", you sure did plenty of assuming that the girl had worn "other clothing that was" revealing.