On Mat leave at the mo, and everything is changing in my absence, but here goes.
Kids are meant to make two sublevels of progress in each year.
Generally, at least in our school, year 7 tends to go from 3 (poor), 4 (average), 5 (very good).
Level 5 is meant to equate to a C at GCSE and we tend to find that a child on level 5 in yr 7 will be a high achiever at GCse (A*/A/B).
However, we do tend to find that a child on level 5 in primary school (or even worse, a level 5 in SATS), isn't necessarily a level 5 in high school. To begin with anyway.
The high school will probably look at your dd's primary school levels and set an expected level from that.
When she gets to KS4, there will be FIsher FAMily TRust data which predicts, based on family background and primary school data, the GCSE grades she should get. The school may or may not agree with that, depending on how well she's performed in the preceeding 3 years.
Hth, because I have a feeling I've confused myself.