Former head of core subject in a very large comprehensive.
Both the SATs and CATs test scores are used as data which inform the flight path for the student. Also attendance percentage for Y6. How much weight every school and every head of department give to each of those data points and any additional points added is unique - data analysis is very much a strategy and is not a standardised process across the country, though there are obvious examples of good and bad practice.
In my school, we also tend to get a one sentence comment from the y6 class teacher that can often hold far more weight. All sorts can be shown in that one cell on a spreadsheet: ‘scored surprisingly high/low’ tells us the child is different in class attainment than in exam conditions, ‘must be encouraged to write’ for a child who is actually quite able but is not self-motivated, along with things that might be relevant pastorally, like they have trouble with friendships or are moving up with a student they could really have done with a fresh start from, and sometimes a note of whether a child is known to be tutored or parents don’t believe in homework or whatever thing they feel compelled to report. Often it will just say ‘no problems’ for a typical child. Not all primary schools do this (state schools cannot require extra reporting), what they say is vague at best, and not all secondaries pass it on (some prefer everyone to just be a raw score and have a completely fresh start for the rest). But at the most, this is the data given to subject heads at the start of year 7.
Equally, subject heads know all the faults of the testing used. So if a kid gets an unexpectedly low SATs test score (every child who ever sat that test with a fever or stomach bug, I see you!) and work at a level higher than that straight out the gate in the first half-term, it will (should be!) caught and setting and expectations adjusted. Equally if they aced it by sheer dumb luck or start year 7 with a dreadful attitude, they will probably move down a set after Christmas.
As for GD scorers looking like they are not doing well, it’s just a case of adjusting to a secondary marking mindset. Especially when you get to GCSE - the first marks of year 10 will look rubbish purely because they haven’t seen all the material yet, so if they went in to sit the final papers that day, they wouldn’t be ready. We aren’t giving an imaginary grade of where you would be if you keep working at this exact level, we are telling you where you would be if it was all done right now. Obviously it’s not all done right now, so you will move up! Not all schools are entirely consistent on this either, so ask for clarification if you are ever unsure. The teacher will be able to tell you.
But if you achieved GD on Year 6 SATs and you have decent work ethic and attendance, you should be achieving excellent results across your subjects at GCSE (and yes, literacy really does impact success in other subjects that much) so don’t get bogged down in data analysis. That’s someone else’s job. Yours to help keep that student on track so you get the same sort of happy exam results at the end of year 11 and 13, when they really count!