"So when a school has 80% of its children taking KS2 SATS having English as a second language, having arrived in England - and that school - since the age of 7 is it a failure of the school if fewer than 65% of the cohort reach Level 4??"
Schools are allowed to discount the results of children who arrived in year 5 and year 6. There are schools in London that get over such adversity though.
"Or if 60% are on FSM in the middle of a community with hugely high unemployment?"
There are schools like that that do get reasonable results. The theory is that pupil premium should over come such hurdles.
"Or if many of the children arrived in Reception pre-verbal, not toilet trained and having no concept of interaction with an adult?"
Special schools do not do SATs exams. Are there really many mainstream schools where many of the children are ", not toilet trained and having no concept of interaction with an adult"
In my daughter's nursery there are two children like that and both of them have substantial medical special needs and statements.