I think you should try and make as much of a rounded judgement as possible, much as your self allude to. I would treat the somewhat arbitrary 5 GCSE measure with a good dose of care.
Schools are the first to moan about leagues tables and the bluntness of Ofsted judgements. They are also the first to print ads with outstanding on them and the 100% of their pupils achieving GCSE's pass (G or above) when it suits. High achievers will achieve even in the most atrocious school, but so do middle achievers. Look at Lordswood boy for last year, 2014 - 10% of boys got 5 GCSE's, but look a little more closely at the numbers
There were 96 boys in the cohort as follows
26 low attainers
57 Middle attainers
13 High attainers
Of these 31% of high attainers achieved 5 GCSE's and 11% of middle attainers.
So in practice 4 kids in 2014 (high attainers) and about 6 kids (middle attainers) got 5+ GCSE's out of some 96 kids in yr11, which is the 10% of boys overall who got 5 GCSE's.
The other 85 kids or so did not get nothing, most likely they got something, just not over the 5 GCSE's threshold.
Other notable stats are evident. Nearly 10% of boys are dropping out of education altogether (that is almost as many as those getting 5 or more GCSE's). The way I read the stats tells me, if you not one of the 10 boys that got 5 GCSE's or 10 that drop out, the remaining 75 are trying to pick up their education, by carrying on into FE or a school sixth form, in an attempt to get back on track.
The other dimension is the high % that are classified as disadvantaged, and the need to understand how many of them are making expected progress, in maths and English. It falls off a cliff for those those classified as disadvantaged. Again a difficult one as nearly half don't speak English as a first language.
Many would conclude on the basis of the same stats that either the school was doing a great job in difficult circumstances or that in fact it was a basket case. Take you pick
Trying to prevent this turning into war and peace - let me pose a series of questions that you could ask yourself
1) How much skin in the game does the head have? If I am sending my kid to a school is the head's time going to be split, between a failing school, thus dragging down the performance of the good school.
2) Is the head angling for retirement and taking on Executive responsibilities at the same time, if so why?
3) Is there a discernible trend in performance up or down
4) How good is the governing body?
5) What can really be deduced from the stats? You said 97% of high achievers getting 5 GCSE's, but in this case high achievers were a relatively small proportion of the overall cohort (and even then they could only get under a 1/3 of those over the 5 GCSE's threshold). Look at both the % and the absolute numbers and proportions, to get a better feel.
6) What does the latest ofsted say to help you get a more subtle understanding of the stats.
7) Look at the school, is it the sort of school, that on the first day. is going to start telling kids what will happen to them if they don't tow the line (the sort of school, that has its starting point, that the kids coming through the door are out of control and could well have a police officer on site permanently) or is it the sort of school that will set out a clear ethos of growing a child's self esteem, grit, resilience, self confidence and leadership skills.
Bottom line...... Stats are important, but only you give a part of the puzzle. I would not find it easy to give up a grammar in Birmingham (if I had a place), given what they stand for and what they achieve. Most of the parents that send their kids to them are likely to fight tooth and nail to support them and their children, which cannot always be said elsewhere. Even in primary you can see a clear line emerging between those that do and those that well