This is actually a really interesting. As with many fads in education, there is clearly a validity issue with flightpaths.
Also just FYI A level grades are predicted/judged from GCSE data. A score is derived from GCSEs which is then used to predict A levels based on previous cohorts (e.g. 60% of students with this score at GCSE obtained a C at A level in this subject). Again, schools are judged against these national statistics (and this is where "progress" comes from - and helps the government create league tables that you can access online on their "compare school performance" website). The higher the progress figure, the better the school's score against national data.
It's an interesting debate because, on the one hand, parents want to know how well schools are doing, and quantitative measures of progress are often fairer than overall measures of attainment as these take into account approximate starting points for the children and are more likely to be able to give an idea of whether children can achieve their potential, as opposed to, say, coasting grammar schools.
On the other hand, you get the "guessing", sorry, "professional judgement" of ks3 progress measures, with all teachers under pressure to show that their classes make good progress. This means that there is highly likely to be bias in the system (automatically inflating grades in-line with their predicted progress - in many schools this might mean a mostly green/yellow spreadsheet rather than a mostly red one) until the music stops at the end of y11, with whoever is then teaching them left accountable for their results.
I'm doing a Masters in Education at the moment and have to do an essay over Easter. I was going to do it on implementing the new RSE legislation, but I might do it on measuring progress instead!