Just let her do that... and per her experiencing it and it being a "hassle" she may realize, that SHE needs to make her own decisions and not just do what her friends tell her. SHE needs to do, what is best, for HER... not just going by what everyone else is doing and to have her own mind... and not just do things according to others.
Yes, you are right per the points you made about HOW will she carry all that, her back, and her flute, and if it rains etc.
You have the common sense, she does not.
But she is 17 and "determined."
Have HER pay for it.
And yes, her back AND shoulders... WILL get sore from carrying things like that.
Have HER PAY FOR IT.
She must have money for herself, by now. She is 17.
I've used messenger bags. It hurt my shoulders and back.
As women, we carry our purses on one shoulder and with TONS of things in it.... and that can hurt our shoulders too. But... a school bag will be MUCH heavier that a woman's purse.
Your daughter needs to learn about her mistakes.
AND PAY FOR IT HERSELF, if that is the kind of bag she wants.
And, WHY have a bag.. .if she says she won't put *all* her stuff in her bag... but will carry it in her arms instead. Plus her Flute and a jacket over that, IF it rains. She will end up... having ALL her stuff, just fall on the ground.
Your daughter is just wanting to do this... because... a friend told her to. And told her that it is "cool" to have a messenger bag.
So, that is the other.... problem.
She is doing what other kids are telling her to do.