My mum was a teacher and tought in a few schools from CoE to what was the worst school in Kent and in the bottom ten in the UK league tables.
Mum always said her best time at teaching was in the worst school in Kent. This may sound odd.
But in the local grammar school the kids came in expecting to get 8 or 9 top marks, which they kids did. At mums school the kids came in expecting to leave with nothing. 95% of the kids did leave with some quifications.
So which school improved the kids ? The school were kids were expected to leave with 8 or 9 and did, or the school were kids were expecting to leave with nothing but left with some quifications.
Mum was always proud to get this kids through to some quifications, these kids came from the poorest areas etc. The grammar school kids didn't.
League tables don't tell the real work that goes on to improve kids it just goes by exam results. So if the school cherry picks all the 'best' kids it's top