I think that she is at that age where holding her back will have detrimental effects that you may not be able to conceive. She will become an outcast by her friends when she is finally with them again, she will be cut off from everything.
At this age she will be harmed more by holding her back.
If she truly has real issues that are a direct result of the diagnosis then you need to get her on a 504 plan or an IEP. They are responsible for providing assistance for her, to make sure she has a successful educational experience.
This is what I would do, and yes, I am in the process of doing this myself for our grandchild.
The school day is from Xam to Xpm. They have a certain number of classes they must enroll in. Such as math, science of some sort, english/language arts, etc...
I am asking the school to put my grandchild in study hall at least one hour of the day where she will be forced to do her work while in the school building and teachers are available to help. This will force her to ask for help and to work on her assignments.
She isn't lazy but when she doesn't quite get something or she isn't able to figure it out instantly she stops doing it and just sits.
If she's in a study hall situation where she is to be working on a specific class assignment then that teacher who is overseeing it will know exactly what to do.
I don't know if that's feasible or not but with a diagnosis and a plan in place that says she needs time at school to work on assignments I think they would have to do this.
My hope, serious real hope, is that she will go to this hour of the day with a physical list of assignments she needs to do that day. Including pages to read, links to any online materials that she can access, and a basic outline of how to do the work.
Yes, it's like working with a little kid but with her mental health issues right now I think we have to be on board with this. She flat out won't do any school work at home. I can ground her, take away electronics, take away activities, parties, events, friends visits, everything...and she just doesn't care. She will not do any work at home. She hasn't had any teachers until this year that want things done at home. Not any homework.
Nearly every teacher she's had, below middle school, have absolutely not wanted the kids to do work at home. Parents often screw up how the teacher wants things taught or they don't have the ability to do that level of work. I mean really, how may of us can do math the way they want anymore? I know math teachers that get their books and stuff at the beginning of the school year and they go through them and can't figure out how the authors got from A to B in the problems.
So we're battling with similar issues.
There is no way in heck I'd let the school retain a child this old. It will be extremely detrimental to her emotionally and socially. The Department of Education states that nearly 40% of retained kids do NOT finish high school. Seriously? That many kids that were held back just dropped out at some point> when they turned 18? or just whenever they felt like it? No way I'm taking a chance.