It very well might be innocent–she’s showing off to us far more than she is to the teacher. If the intent was to show off to the teacher I don’t think she would have a top that came up so high in front.
Trust me, this sort of thing happens all the time, at a shockingly early age. Most of it is innocent, at least at first, and if not, is often aimed, as Loren eludes to, not at the instructor, but at another member of the class.
But, honestly, many older girls and young women are actually completely unaware of themselves, if you see what I mean.