Given the ENORMOUS amount of effort many schools already put in to get Y6s to the 'magic' L4, including a wide variety of interventions, I cannot see how a secondary school is going to make a huge difference within a short time.
I mean, many children's Year 6 levels, from many schools, are already 'maximums', rather than 'normal working levels'. For a secondary school, from scratch, to take those children from where they 'really' are to a L4 equivalent in a few months seems a very big ask ... unless the powers that be think that primaries are not very good at their job and a secondary, despite doing less english and less maths each day than at primary, can suddenly wave a magic wand?
Internal marking of what will become a very high stakes assessment could also mean .. well, to put it politely, quite a significant level of moderation....