Look at the Value added, and ask each school about their SEN department and how many per year have SEN, and what results they achieve, then ask about their G&T pupils, how many they have on average per year, and the results they achieve.
On paper, my local Secondary (almost fully Comp, except the highest lvl 6/7's will have gone to the superselective Grammar) has a not so good value added, despite being 'Good with Outstanding areas'.
In practice, even their SEN pupils are achieving 5 A*-G's. and most if those will be D and above, despite pupils like my DD starting that are 4/5 years behind their peers.
Their SEN Dept is amazing.
However, very few of their G&T pupils achieve a clutch of A*'s.
So that's why their Value Added slips down.
Best school in the town if you have a DC with SEN's, best school in the town if you have an average DC, not so great if you have a high flyer.
I personally think Value Added combined with knowledge about which groups of DC's do well FOR THEM in the school is the best measure of a school.