There can be a huge difference in the average intake from one school to another - a school can take in a high ratio of bright children with loads of home support, get high SATS and still actually be a crap school, or it can take in a high ratio of children with low home support, low ability, be a great school with excellent ethos and teaching, and have an 'average' outcome from a below average cohort. Many parents wiloll flock to school 1, even though school 2 has pushed children further.
And within all that the statstic that tells you most about how an individual child will perform is perhaps the Value Added.
Within any of the averages, there may well be a couple of children fulfilling high potential to a spectacular level, or a few who are well below average.
Have a look at the level of free school meals in different schools - it varies wildly - and is an indicator of economic disadvantage. However, two of the highest performing kids in DS's class are eligible for FSM. Shrug.
IMO the hysteria about underperforming schools is overblown. We should instead be concerned about the specific underperforming children due to unsupported dyslexia, or parents who never get them to school on time, or those being bullied.