That isn’t ‘normal’ teen behaviour ime.
Saying about her feeling stressed about peoples elbows touching hers stands out. I am NOT doing an armchair diagnosis but there are 5 DN’s (mix of girls and boys)with ASD and/or ADHD in my family and being intolerant of people being in her space/touching her, not being able to prioritising tasks, stressed, tantrums & violent outbursts does seem like a lot of behaviours and triggers that they all have. It may be worth exploring, especially as you say she acts normal at school. The effort of masking at school is often exhausting and causes the behaviour to fall apart at home.

My first thought was some kind of neurodiversity too.

Neurodiversity diagnosis isn't going to help her in the short term though. Even if she was diagnosed tomorrow, these issues will still be there.

Do you think it would be helpful to her if you both worked out some strategies/behaviours which would break down some of the overwhelming tasks into easier, smaller, tasks?

Would it help her if you came up with a phrase she could use at school for the "elbows" situation?

I think a lot the time, even if you don't find a solution, just talking it through can be beneficial.

Good luck - my teen was very anxious at this age, but we did somehow muddle through by talking and listening. Listening mostly, from me.

It's not an easy time, and a lot of stuff is "teenage angst", which isn't to be dismissed, but it's part of brain development and growth. Keep talking OP, you sound very supportive and I'm sure that's helpful.

How worrying for you OP. I don’t know if there’s any way you can help her get more rest/sleep- get into Pjs much earlier in the evening before a treat like hot choc or favourite to programme or something? How is she eating - plenty of iron rich foods, protein and veg etc? Does she need reassurance re school in terms of how she’s doing ? I might be tempted to speak to teachers to see if they can move where she sits or keep an eye out for any unfriendly behaviour at the tables?? Is she working too hard at homework every weekend, does she need a weekend off doing something relaxing? If there’s no improvement I might ask the GP for some blood tests to rule out whether she could be anaemic or something

It’s not easy is it. You’re trying to help her but she’s getting upset and angry when you do. I suppose she is at that age where she wants to control when she does her work etc. I’m happy to hear she’s got a hobby she enjoys, that’s a plus in life whatever happens! I wish I had more answers for you. Perhaps all you can do at the moment is try to take little opportunities to reassure her and show her you love her.

Which I’m sure you are doing already

Would she maybe go out for walks with you ? Sometimes it’s easier to talk to teenagers if you are ‘side by side’ doing something rather than face to face. Or doing something else together ? Perhaps try doing something together and specifically avoid topics of concern so you can try to relax together and take the pressure off?

I assume she's in y11 and has GCSEs soon? My DD is too and is struggling this term. Mood definitely low, and very snappy and tearful, especially if you mention school work or exams in any way. It must be so stressful for them.

I could have written this last year when my dd was 15. She has Sensory processing disorder and tends to be anxious but she coped fine in Primary and with the initial move to secondary.
I feel she was quite badly affected by the pandemic and enjoyed home schooling more than regular school.
Last year she had some big exams coming up. She became really stressed. She was struggling with friendships and became very stressed about day to day school life.
She ended up refusing to go to school for a time. I had some support from the school and also arranged counselling out of school for her. The school recommended she be assessed for ASD but we haven't had that done yet.
She didn't think she got much from the counselling outside school but felt very supported by the in school counsellor.
She managed to settle back in to school and got through her exams.
We aren't in the UK so different school system here. She has a more practical year this year, less academic pressure and she is so much better.
I think your dd would benefit from having some support at school but I understand it is difficult to get her to see that.
Dd didn't want me to speak to the school at first but once the school refusing started I had no choice.

I wonder if you can work with her to talk to school together. I would imagine that she is having lots of worries about standing out / being judged when actually there will be countless kids that are seeing camhs/having support from school that she is unaware of.
That opens up care/support in school but also thinking with them about inward referral if your gp is asking for school involvement for onward referral.

I think this is an exceptionally stressful year.
If you haven't previously noticed any behaviour that pinged any concerns, it probably is just very heightened stress with a large dose of exam anxiety on the side.
She's 15, a high achieving and able student. Most schools do offer some pastoral care or support but if she refuses to engage there isn't much you can do.
I find my teen easier to talk with when we're doing something like going for a walk, it's easier to open up - maybe ask how you can make home a better place to unwind and decompress. Stress and procrastinating go hand in hand.
I don't think that there is a magic solution (please tell me if you find one!) But give her a space to vent and air her concerns and frustrations. I think trying to help teens organise themselves is like pushing water uphill..

Ah you've got a while to go yet then. My DD didn't do mock exams until y11, I think it made her y10 a bit less stressful.

With my DD I just try my best to make her weekends and holidays enjoyable and relaxing as possible. I have stopped mentioning school work, it wasn't helping.

I listen to her rant about school, life, whatever and don't give advice or pass comment unless she actually asks for it (which she doesn't). I just say "there there" type things like "that sounds hard" and "poor you". I learnt that I'm the person she dumps all her worries, anxieties, anger and stress on, and she just wants listening to and comforting. I have to remind myself shes just offloading, and not to go into panic mode. I think she feels better afterwards, and has often moved on whilst I'm still worrying about it!

ADHD can make simple tasks seem overwhelming. I've been known to put off going to bed until 3am because I can't face the task of getting ready for bed. It sounds ludicrous but it's just how it can be.

Girls are often missed re ADHD because it's still considered a condition that relates to naughty hyperactive children but it presents in a less obvious way.