Well, that is terrible. Sorry to hear. Starting school is hard enough without an armpit yeast infection. Does her doctor not agree that it is yeast? I believe they can swab that or do they visually inspect? I would go back. Diflucan is something babies are given. Why wouldn't they treat it? That's really kind of ridiculous.
Things you can do to treat it yourself are tree tee oil. Get this at a health food store. Coconut oil may help and oregano oil also may help. Is her school air conditioned? If so, good. If not, work with her teachers and have her sit in front of or under a fan (preferred seating) to help cut down on sweating. Light weight cotton clothes, loose fitting only.
I'd also ask her doctor to do some lab work to make sure she doesn't have any issue with her sugar or is prediabetic. There is a greater instance of underarm yeast when that is the case.
Anyway, I'd be really loud at the doctor. Insurance or not---- pay cash for a one time visit. or get some tree tee oil. Let us know how it goes hon.