Just looking at the SATs statistics for last year of schools local to us and looked at the school I removed dd from in y1. The ks2 stats therefore relate to the y2's that shared her class. So it says 100% 4b and above and average score 5c.
It's a small school so average year size is 12, back then I used to help out in class along with concerns about the teacher's treatment of my dd (didn't seem to grasp autism/SEN etc) I was also worried because many of the class couldn't read (dd was probably more able than most of y2 and all y1) and lots couldn't spell even the most basic words and some not even their name.
So is it possible (well it obviously is according to statistics) that all children made what would have been miraculous progress for some to average at 5c four years later?