As others have said, the average across the country is 45%.
A previous poster provided a link to the compare school performance service gov.uk website. This is a great resource to compare schools in a local area.
The average across all schools for the Local Authority where I live is 36%. This ranges generally from 17% to 50% with one school at 79%.
The school that has the very high figure is a Church of England secondary school that has around 10% of places awarded based on passing an entrance exam or on musical ability.
The rest of the places are given to children of worshipping members of the Church of England or Methodist church. After that it goes to children of other Christian denominations and Other Major Faiths.
Only if there aren't enough pupils in any of the above categories will other local children be considered on a nearest to the school basis.
And they expect the parents to have been active, regular, worshippers for a number of years. You get points depending on how often you attend and for how many years. You are then ranked and those with the most points get offered places first. You can't fake it either, you have to get written confirmation from your vicar, priest, rabbi, pandit or imam.
The school is wildly oversubscribed and parents (unless they physically live very close to the school) plan several years in advance if they're planning for their children to attend this school.
If you look at the breakdown of the figures for this school on the gov.uk website (look for the school and then click on Secondary, Results by pupil characteristics, prior attainment) it gives figures for different groups of pupils based on how they did at Key Stage 2 - they are referred to as "Low Prior Attainers", "Middle Prior Attainers" and "High Prior Attainers".
For example, this school in particular had 5% low attainers, 45% middle attainers and 50% high attainers.
So they had a lot of very bright kids going to that school anyway to start with. But it also did great when it came to teaching those children and the Progress8 score was also very good as well for all groups. Also, they only had a very small number of disadvantaged pupils (8%) but they did well with a 46% pass rate.
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Contrast that to the school that came out bottom (excluding special schools etc).
This school had a 17% pass rate. But look at the type of children at the school. 48% disadvantaged. When it came attainment there were 45% low attainers, 45% middle attainers and only 5% high attainers.
But despite the overall low scores at that school those high attainers still managed to get an 85% pass rate (the pass rate for high attainers at the top school was 93%).
Having said that, their Progress 8 score was definitely down at the bottom end so there is definitely room for improvement.
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So, find out where your DC are in terms of Key Stage 2. Are they low, middle or high attainers? Are your DC disadvantaged or have English as an additional language?
Then have a look to see how each secondary school is doing for the relevant group for your DC rather than trying to compare an overall figure that may be very different simply because there are more high prior attainers at one school than at another.