Hi The Fox & the Hound, real kudos to you for caring so much about how this girl is feeling
Do you know any of her friends? If so you might want to mention to one of them what you've noticed to give them the opportunity to offer her some support.
If not, otherwise..........you might want to mention you've noticed "something seems to be getting her down" and if she wants to talk about it..............
But if it starts getting "too deep" for you, you do need to point her in the direction of support e.g. school counsellor and even talk to them on her behalf. At least then maybe you'll have done more for her than some of them around her, maybe more than all of them!!!
And back to YOU The Fox & the Hound, are YOU getting any help now?? You know we'd all be wanting you to be getting the help/support you need.

Alison