The rankings are the key thing. They will submit a grade and rankings within the grade but the ranking is key.
If Ofqal decide that according to their formulas, 20% in school X should get an A in a particular subject and the school has given 24% an A then the lowest ranked 4% given the A* by the school will go down to the A grade bracket. They will then be the top students in the A grade allocation.
It is absolutely the right way to do it. The ranking will be within the whole school year group and not just within a class.
Schools which historically get high %s with high achievement will get it again this year.....as they would have if exams had been sat.
Some students may end up with lower and some with higher grades than the school said, based on the number crunching to achieve parity across the whole cohort in the country, with previous years.
Most students do not achieve their UCAS predictions in exams and won't this year either. Most will still go onto uni and it will probably be a buyers market this year so lower grades than usual will be taken, and bearing in mind many Unis take students with less than their offer (even the good ones and even competitive courses) most students will be fine.
When might you lose out? If you have been right at the bottom through the whole course and that was a consistent under-performance in relation to your ability. We are not talking one poor essay or one poor mock exam. If you are ranked right at the bottom of the year group, whatever is the schools usual lowest grade, is probably what you will be given, unless the year group has shown itself more able than previous ones. Of course it's unusual for someone who has consistently been at the bottom of the year group for the whole of 18 months to suddenly pull it out of the bag in the exam, but it is a possibility and that student won't get a chance to do that this year. But they will be able to sit an exam in the autumn to show what they can do if they feel the grade they are given really doesn't reflect what they would have achieved. In the vast majority of cases, it will deliver something which is very accurate and similar to what the exams would have delivered.