I cannot for the life of me comprehend why anyone still uses SATs papers and levels in KS3. In my experience where levels are used to judge pupil progress and thus judge teachers in KS3 pupils get decent levels (whatever that means. Depends on the targets) and then there is a precipitous drop in performance in Year 10. SATs papers are in no way similar to GCSE papers and are indeed poor preparation for GCSE. Ks3 levels are a poor predictor of GCSE grade in my experience. Worse in fact than KS2 levels.
If it were down to me I would use GCSE past questions, papers and grades from day 1. Failing that I would use GCSE stuff from the start of year 8. I don’t see the point of levels any more now that there are no more Year 9 SATs.
Should people wish to continue using levels for some reason then they should be used in a useful way. I looked at historical results to map out every path pupils had followed from Ks2 through every end of year assessment through to GCSE result. This enables me to say people in this school progressing as you are go on to get a D. You need to improve if you want to pass. Pupils and parents find that far more meaningful than 5b, target level 6c, underachieving. Many of our parents have no idea whether 5b and 6c are any good or not. How serious is it if a child is 5b when their target is 6c?
Using SATs papers or Teacher assessment is KS3 suffers from the same problem as KS1 exams. Teachers are being judged on these levels as a measure of pupil progress. If teachers are marking the papers and giving levels based on their professional judgement and judgments about their competence and performance are being made on the basis of these levels then the temptation is always there to inflate results. This is one cause of the drop in performance in year 10.