Starving themselves

So recently i was at my nieces school looking over the class (because I'm a volunteer) and they split every one into groups. Now in the 7th grade girls really care about there body it's like there prime time, so one of the girls from my nieces group asked for help I came over politely and she finished her statement and as we were talking she got comfortable and quit sucking in her stomach before she came into class she was sucking in her stomach so hard that you could see her rib cage all girls do this except for my niece they decide to suck in their stomach to show off their curves or show that they're skinny so like I said before she got comfortable and stuck out her stomach as I left the group this one girl who is next to the girl who forgot to suck back in her stomach started laughing and laughing and laughing at her as she asked what was wrong she realized that she forgot to suck back in her stomach and slowly sucked it back in acting like she had no idea what she was doing now I take great offense to this because these girls should not be ashamed of their bodies if they're fat they're fat and skinny they're skinny they don't have to be ashamed and sometimes it's worse I've heard of some girl pressuring them to throw back up their food and it's not healthy at all give me some advice so I can know what I have to do.

Do I tell the principal do I tell the girls to stop what do I have to do