I have worked in CAMHS.
Begin by asking her what helps her to feel better and how you can help with that. If he is a reader, I would definitely look at Young Adult books, and do think about 'fantasy' books that can often allow readers to think more widely.
Pursue all the avenues above. School must be told (at least something) and they may have referral options. They may suggest a referral to the school nurse, who may themselves be trained in dealing with mental illness, or a have a referral pathway.
Ask for a CAMHS referral because even if the waiting list is long, many departments have useful links or ideas.
Young Minds website can be very helpful.
Do consider, if you can afford it, a private referral, but please use caution. Young Minds has a helpful section. Counselling qualifications can be very variable, and some are not experienced at working with adolescents.
In the meantime, don't underestimate the value of 'just being there'.