I'm not sure what the results on their own are going to tell you. My DS1 went to an 11-16 comprehensive locally to us, and applied to do the same combination of courses at 5 sixth forms (including three colleges and 2 school sixth forms, one of which offers a lot of places to external students). All offered him a place and he was clear at the beginning of last summer that it was between two, APS school sixth form and Woodhouse College. He did comment that one of the others, that markets itself on status and Oxbridge/Russell Group entry, actually seemed to get lower grades on higher entry requirements.
In the end he chose APS. He has said sometimes that he wishes he'd gone to a college, but the mum of one of his former GCSE classmates whose son is at Woodhouse has said her DS is struggling a bit and she is clearly not that impressed by levels of support.
If there are a number of places to choose between (or other options) it can be a difficult choice but I'm not sure about comparing grades separately from culture. Can you compare grades of those who did both Maths and Further Maths, or are Maths grades for everyone who sat the A level Maths exam? What are drop out rates like? (bearing in mind that at some schools like the one DS1 is at, most students are studying 4 subjects in year 12 and choose 3 to carry on with in year 13 - however, it's different for those who sign up for FM). How do they encourage students who aren't quite fulfilling potential? (My DS1 had to be pushed a bit, but I think this was quite successful, so far).
At APS, intending Further Maths students prepare for and take A level maths in Year 12, so DS1 is waiting for his result on Thursday. Then they do Further Maths and 2 other A levels in year 13. Most other APS students do 4 subjects in year 12 and continue 3 in year 13, but Further Maths students are expected to commit to doing a much more intensive Maths course.
Does your DC have particular aspirations and what support is available for students who want to apply to particularly competitive universities or subjects? My DS1 is interested in Oxbridge and he has extension lessons at Highgate (private school outreach, offered free to some students from a number of state schools). He went to a day school event for students who want to continue with Maths, run by UCL, and has a Maths summer school coming up with Highgate before term starts.