It is inappropriate, but not so much because it's sexual as it's the kind of comment that increases self-consciousness and body embarrassment in the shy and emboldens those, both male and female, who like to tease or bully.

It's the kind of rookie mistake young teachers make because, amazingly, they may still be feeling and thinking like a kid themselves. If someone does report it, it will actually be good if an older teacher or the school principal has a private word to warn her to cut it out. If she continued after being warned, then, yes, she'd probably be in trouble. It's not at all uncommon for young people just starting their grown-up careers to make these mistakes and to require some guidance from more experienced professionals.

Please don't spend too much time worrying about it. If she gets in trouble, it's likely to be just a little trouble, and it may set her straight so she can have a long and happy teaching career. We all make mistakes when we're starting out and her mistake was forgetting to set the right example.