Could there be something hormonal at play, especially with the tearfulness, or some kind of problem like a fall out with a friend or bullying? Combined with exam stress would be an awful combination. Has she got plans sorted for after exams such as going to college that she is worrying about, not making the grades (feeling like her whole future depends on this), second guessing her plans, being separated from friends, even if it isn't anything logical or rational?
I'm not sure if it's suitable for non-adults but could something like Bach's Rescue Remedy help, something to give her a bit more of a crutch, even if psychological. You can also find guided relaxation on the internet if any of those could help her, such as switching off to sleep after revising. Is she getting enough sleep with revising and exam stress?
Maybe organise a nice treat or series of things for her to do once her exams are over, not associated with a reward for having done well, but things which might help her relax like a spa day. Something to look forward to and something to help remind her that there is more going on than the up and coming exams, which can be all consuming.
Is there anything she is in to, or might be, that could help give her something to channel her emotions? Something she enjoys but there might have been little time to do with school? Something like running or swimming?
Did you go to the doctor with her last time and, if so, would she consider going in on her own? Might help her open up a bit more if she doesn't want to worry you.