I can see why you are concerned! It must be very frustrating for you and I remember answering on your previous thread, although probably under a different name!
Writing is such a subjective area that it is sometimes difficult to get consistent levelling. Is the reading moving forwards or are you just getting one overall level for literacy?
Could you approach the school and just say that, bearing in mind that your dds progress has levelled off (presumably she must have been making at least 2 sublevels progress and clearly sometimes 3 or more at primary) you would like to support her at home. Then ask them for the levelling grid. It should show what critieria your dd needs to meet in order to reach the next level. Then you could ask what sort of activities would support this. What comments do you make at parents evenings?
Has she had targets to meet? At secondary in particular, your dd should be familiar with targets and Ofsted like to test this with children. At many secondary schools they sit CAT exams or similar upon entering and this gives targets for achievement at GCSE. With a L6 at primary she should be on target for A*. Her progress or (lack of it) just be tracked so that the school/Ofsted/Governors can check how they are adding value.
What I cannot understand (in your case) is why the school are not concerned. They track progress with nice pretty graphs and your dds case will not look good. It would be a 'red flag' for Ofsted for how bright children progress. They have either amended her starting level on their system, but not, clearly, on the reports or they are an incredibly 'laid back' school and are explaining it in some other way.
I would ask the school to show you their progress tracking system for your DD and ask for a copy.
Are there any other schools in the area that stream that might be more appropriate for DD? Although many schools say it is easy to differentiate for ability in literacy, I am personally not convinced. We have had similar concerns, in the past, regarding DS who hit a high level 6 in yr 4 for reading, although the writing lags a little. What we found is that the reading storms ahead, whether 'taught' or not, but that some aspects of writing do need to be taught.