I imagine there won't be any laws about it, nor will they force her out of the school. But they may well pressure her to redo lower sixth, I recommend being extremely clear to them about your wishes and make certain that they are aware of any extenuating circumstances. Furthermore I'd also recommended discussing other solutions that won't be repeating year 12. But please don't worry or punish your daughter, it's all ok. I used to be in a similar situation where I got DEU in my lower sixth mocks, my sixth form tried to bully me out so I wouldn't lower their averages. I ended up progressing into upper sixth, but at the same time I was forced to do my lower and upper sixth classes for my worst subject. That case was quite common in my sixth form with 20 others in the same circumstance. I was very ill in year 13, so I got CDE. However this year I've taken 2 entirely different A-Levels in one year that I self taught and I got A* A*. So I hope this might offer some hope, and reassurance that you're not alone. I just really recommend being your daughter's biggest advocate for what she wants and what is best for her. I hope all goes well.