Please trust that the teachers will be doing their best. They'll probably be agonising over the ranking, because it isn't easy, but they will try and be as fair as they can.
In a way, it's hardest with the largest cohorts, because very little may separate 15 kids. They'll be using all the information they have - mocks, progress since then, etc. When it gets to the stage of coin-tossing between pupil A and pupil B, I suspect mocks may be the decider, just because that's the most objective thing, but that doesn't mean that pupil C (who did worse in the mocks but has made masses of progress since) won't stay above them both.
If your child is one that is bumped down (or up) a grade, it's only going to be one grade. Schools' should be able to say with reasonable confidence whether each student is 7, 6/7, 6, 5/6, etc.
Everyone is going to know that this cohort's results may be a little more random than usual. Colleges will probably bear this in mind when admitting people to A-level courses, and might waive particular requirements that they might normally insist on.