At wit's end with DD's behaviour and poor school performance

You have described my daughter exactly - very bright at primary would do pretty amazing things and was an excellent sportsperson as part of a competitive club. Secondary school and everything went downhill. Gave up all clubs, behaviour nosedived, ended up in detentions every week and obsessed with hair, makeup, uniform and breaking school rules.
For her it’s quite complex, she is diagnosed adhd so is very impulsive which make her argue with teachers and do silly things in class, especially if she thinks others will find it funny. She also struggles to focus and concentrate, would walk out of class and goes to the toilet a million times a day to get out of sitting in class. I think boredom is part of it too, but not that the work isn’t challenging just that she’s not interested in a lot of it. And like another PP said, I think she’s having to work harder to keep up, whereas she sailed through primary not really having to work very hard.
We’ve tried every sanction, consequence, nothing has really helped. She just has no respect for rules or boundaries at school or at home. We’ve talked to her at length about how this will affect her future and how she’s wasting her talents but nothing seems to go in.
One thing I will say is that things are slightly better now she’s in year 10. I don’t know if anything we have done/said has started to sink in or whether she’s just grown up a bit but her attitude has changed a bit and she’s also on meds for her adhd which helps a lot too. Also she’s not being forced to sit in classes doing subjects that she didn’t want to do (boring!) now she has made her gcse choices and I think this has helped a lot.
Hang in there, I know it’s easy to say and it’s so stressful. The only advice I have is to keep talking to the school, you’ll probably find they don’t think of her how you think they do (she always has great comments at parents evenings).

she's had at least one detention per week since she started because of uniform violations - tie is too short, skirt is too short, not wearing blazer, etc. Basic, stupid stuff that is so easy for her to remedy but she persists on pushing the boundaries
Is the uniform issue because she’s generally messy and disorganised, or is she doing it to provoke? Maybe she doesn’t want to be seen as a geek and is acting up a bit to seem badass and be more popular.

They’re all different at that age, but detention is socially embarrassing at my DC’s school, so there’s a peer pressure to be cool while still doing well.
I honestly can’t see anything wrong with your DD. She doesn’t vape, drink or have underage sex, she has friends, and she’s bright and creative. What really is the problem?

How long are you going to insist on the extra curriculars? When she is 16, 17, 18, are you still going to make her carry on with things she no longer wants to do? If she's telling you loud and clear she wants to drop something then I think you need to listen - she needs to feel that she has agency over her free time.

Otherwise I think a lot of her behaviour sounds normal for Year 8 - it's a bit of a dead year in the sense that everyone has found their feet after Y7, but the more serious stuff (GCSE options etc) is still quite a long way away. Keep imposing boundaries and consequences for disrespectful behaviour, but you have to chalk some of this up to her age.

I think you are putting too much emphasis on the intelligence thing. She won't be the only clever child in school and bright children actually tend to be more engaged in school than the weaker ones. Bear in mind, most of the teachers will teach their subject to A level and will be more than capable of stretching someone in year 8. I would be wary of using that narrative with her because it puts pressure on children. Some avoid putting effort in so that they can blame any lower marks on lack of effort rather than risk losing their reputation for being clever. It could even be that she is resentful towards school and teachers because she hasn't done so well in some subjects or hasn't been put in the higher set for something.

However, it is probably just hormones. It is very common for teenagers to change interests, develop a bit of an attitude and become more interested in their social life. Her behaviour sounds pretty normal. Most teenage girls grow out of it by Yr10.

@moppingtheboard My dc went to a fairly swish girls boarding school. They knew girls! No stupid ties. Blazers - girls decided if they needed to wear it! Short skirts - of course! Yes, the odd telling off but certainly not detention. This school is controlling over stupid things. My old school didn’t care much either - and that was a grammar. We had to wear uniform on leaving the school but it was fairly relaxed on the inside.

This dd is fairly normal. She’s bored. The lessons are almost certainly not fast paced enough. Nothing to do with Sen, it’s a teaching issue. Such a shame there’s no grammar.

To be honest, most uniform rules are simply idiotic, and if you are subject to someone trying to enforce idiotic rules, you start to lose respect for them and all their rules. It's fairly natural. (Just imagine someone trying to monitor the color of your socks in the workplace. What would you think of that person?) If the school wants to fight this battle, they can, but I wouldn't get involved with that at all. The other things you can try to discuss with her. Personally, I would let her drop the martial art if she can keep fit in other ways.