@PennyS , I don’t know why, but I have been thinking about you and your niece. First of all, she is really lucky to have you – aunts can fill that special “mom/adult friend without the parent baggage” role, and it sounds like you’ve taken that on in spades!
I pretty much agree with what everyone here has said – if she doesn’t do the work, she’ll bear the consequences, and that could be a great learning experience. Getting a major ding on a grade at this age has no real ramifications in life. And if she doesn’t wear the uniform/dress code outfit, both she and her parents will get a call. BUT – and this is a big BUT – this is a kid who didn’t feel confident going to her “regular” school who will be going to a new school at a vulnerable age (yikes – middle school girls!) and whose parents are not committed to supporting her but to bad-mouthing the school when things go wrong. To me, letting her fail right out of the gate sounds like it could be the set up for a very bad year! It sounds like she is going to suffer because she is being encouraged to ignore the requirements by her parents, not because she is an obstinate girl who on principle is refusing to do the work.
I think you could do her a real service by helping her understand the lay of the land – it’s always important to read all the instructions, you need to know when things are due, in having made the choice to attend this school you have agreed to their rules whether you agree with them or not, first impressions matter, and the habits that she creates now are the ones she can tweak to be a successful student in middle and high school. She’s at an age where she needs to be responsible for her own work and when it’s inappropriate to blame others when things go wrong. She is also at an age where she may be embarrassed to be the only one who is inappropriately dressed and who is called out for not doing her work. How much better it would be for her to feel good about her new school and to feel excited about this fresh start and doing well!
You could help her come up with a good way to organize (I love many of the suggestions in the book “That Crumpled Piece of paper was due last week) and make it fun by going to buy those organizing supplies (if her school doesn’t provide them.) Her first task can be figuring out a plan to get through the summer book. (And again, I agree that the objective is to simply get it done rather than to do a crackerjack job. ) If you two are close, and it sounds like you are, perhaps she could spend a long weekend with you before school starts to do some of the work (the last few papers?), go shopping for supplies, get some dress code clothes, etc., and then going forward, perhaps you could get together (even by skype) once or twice a week to help her keep up the organizing discipline and address any academic issues. Framed as “I know you can do great in your new school” from her “coach”, you could be a real boost of confidence for her.
I would guess that she’ll have classmates who get a lot of monitoring and help from their parents, so having you there for her could be terrific. This is a tricky age, and it won’t help if she starts seeing herself as someone who “doesn’t get along with school”. But I think your success will have to be with her directly, not with her parents, and you are going to need to persuade her of why she needs to do the right thing and then help her find enthusiasm for that path…