My dd is only 10 so I'm no expert but I read this thread and I think most kid's go through a phase of not liking school, feeling self concious, feeling they don't fit in or look the way they think they should etc it's part of growing up imo.
If I had behaved that way I would have been told in no uncertain terms that I was going to school, infact I had glandular fever for about a year and until I was diagnosed my mum would make me go, in class I was so cold I sat with a coat on and would frequently fall asleep at my desk but I still had to go to school, it was none negotiable and I knew it. Once diagnosed I was allowed more time off until I recovered.
What I'm trying to say is I think what your daughter is feeling is natural and within the realms of normal, the sucide talk is not good obviously so worth seeing the GP if you feel she is serious but I'd be wary of blowing it up of proportion and pandering to the drama because it will only encourage the behaviour especially if she thinks it will get her out of school.
A lot of teens don't like school, they all find excuses or bunk off. Personally I would tell her she is going because unless she is really sick there is no reason to stay off and in the long term it is for her own good.
I would ask her to write a diary which she could chose to share with you if she wants to, or even to write you letters explaining how she feels so she can deal with her emotions and you can help her if she needs it.
Ask her what she wants when she grows up, does she want nice things, somewhere to live when she leaves home, money to go out, own a nice car, take holidays to nice places etc? I'm guessing she does? So she will need to work hard at school to achieve her dreams and gain the things in life she wants. Help her to imagine her life past school, things she can do when she is 18, 21, 30 etc
A few egs that would have appealed to me at that age were...
learning to drive
going on holidays with my friends
meeting a boyfriend
going out with her friends
travelling the world
getting her own home, choosing furniture and decorating it etc
Having missed nearly a year of school myself I can tell you that it did me no favours even though I thought it was great at the time, I ended up leaving school with 1 GCSE and got pregnant at 16 and though I've managed it wasn't at all easy and I wish to god I had re done that year and gained a better education.
I hope your daughter feels better soon. Having been a nightmare teen myself I can tell you I have come out the other side and am very grateful to my parents for being so patient and firm when I needed it.