At my school, we do use the threat of not releasing exam results until all books are returned. This is due to the high number of books that were simply never returned at the end of the year.
as far as my school goes, our year 11's are on study leave and as they take the last exam in English, maths etc they return the text book, get the relevant part of the form signed and once all the form is signed, its returned to Head of year 11 and then thats it!
The results come out in August and will only be released to the pupil at the careers office on the day, written permission has to be given by the pupil if a parent is going to collect, otherwise a large s.a.e is left with Head of year for the results to be posted on.
If you go to collect the results on the day and you've not returned all text books then you can return them there and then and get results or we do hold onto them.
In the case of your dd, you could collect all the texts that she needs to return and take them back to school before the end of term and then the school would have no terms to hold onto her results. Then she could miss the last day, assembly etc.She wouldn't need to go into school to get them, although we have an advice area open for those students who didn't quite get the grades they needed to move onto A levels etc. You could always go with her to get the results, its normally takes 10mins so no chance for any bullying (and she'd prob need a lift into school to get them anyway!)
I wish your daughter well in her exams and I'm sure if the school are aware of the bullying they will be sympathetic and help her and you
BTW we've never had to withold any results so have no idea if the school really can do that, I know that colleges and Uni's can!