I think A-C meant no grades lower than C and in some subjects an A should be achievable. It didn't mean a range of results for each subject and with the mock results being below C, there is concern.
A school in special measures with too many supply teachers teaching exam groups is unlikely to have prepared the children well for exams and these children may well have gaps in their knowledge because they have not had the continuity of teaching required for good progress and results. Children also pick up when teachers are below par and even if revision sessions are offered, there may be misgivings about the quality of them. However, if they are offered, children should go to them and looking at any old papers (if available) is important to work out what you don't understand.
Teachers must assess children on the work they do in class, in tests and for homework. How well they do informs suggested outcomes for exams. It is not an exact science as the stats show but a child getting a D when they have been predicted an A shows a big problem. Exactly what the exams look like is a bit of a red herring as the teachers assess against the syllabus and use their skills to judge and Mark the work completed. If they didn't do this, they could never show the children are making progress. Frequently, in the worst schools, the children do not make sufficient progress, the school do not record progress accurately, and exam marks are less than they should be which is constantly pointed out by Ofsted. Therefore predictions can be wildly inaccurate.
So, find out what she does not know, get the gaps filled, and make a revision plan. Lastly, do the revision!