Poor thing. She sounds a bit traumatised by it. Her confidence has probably taken a knock too, and it is hardly surprising. Ds1 was absolutely gutted when he first broke his arm (that was the time he needed the op to repair it) as he went from one moment being a normal, healthy boy to the next being in hospital, starved of food, having an anaesthetic (which he had had before, so he knew how scary and painful it can be), in pain, scarred for life, and then unable to do much for 6 weeks. The next 2 breaks, although only minor in comparison, just added to his loss of confidence and he is now very reluctant to try new things eg scared to learn to dive as he fears he will dive badly and break his arm as he enters the water. You should expect a similar loss of confidence and short term depression in your dd (sounds like she has the depression already).
That said, I honestly do think she will quickly come round to her experience once she is in the full cast. She shouldn't be in any pain and she should have full mobility back. Even when my son broke his shoulder (yes he has had a depressingly full range of experiences!) and couldn't have a cast but had to wear a sling for however many weeks it took to recover, he still quickly learnt to do what look like impossible tasks by himself, eg dressing himself, showering, even eating with 2 hands.
The other thing to look out for is the attention she will receive once she is back outside the house. My son is very shy and does not like to be the centre of attention at all so he found it really hard to go to school and have people flocking round him, and as for complete strangers asking what he had done - well, it was hard for him. I find that less easy to understand as when I broke my arm as a child, I revelled in the attention, but if your dd is shy, bear that in mind.
Re school: when ds1 broke his arm badly and had the op to repair it, he was off school for a week and a half but that was more to do with recovering from the op. The other times, he went back to school as soon as the full cast was on, or, in the case of the broken shoulder, which he did last Easter,he went back to school when term restarted. I think schools are generally familiar with dealing with children in casts and as long as your dd is mobile and able to go to the toilet/feed herself (which of course she will be) then they will have her in school. I began by asking if my son could stay in at playtimes for fear of him injuring himself again, but that didn't last long as they want to go out and be with their friends. As she is in Year 6, I would say she should be in school as much as possible in the next few weeks, even if she can't write, as they will be doing a lot of revision for the SATs. By the way, I can help you with the question of when SATs week is as ds1 is also Year 6 and it is the same throughout the country: it is the week starting 14th May, so hopefully, your dd will be out of her casts by then.
Gosh, I have gone on a bit! But anything else you want to know, don't hesitate to ask me. I am sadly all too experienced in this way (oh, ds2 broke his leg when he was little and was in a full leg cast so I have even more experience than I had declared!)