53% is below average.
This number tells you that it's unlikely to be the most academic of schools.
It doesn't make it a bad school, though it might be.
You need to look deeper to find out what's going on.
Here's a school regarded as 'Outstanding':
www.education.gov.uk/cgi-bin/schools/performance/school.pl?urn=134693
The first thing to look at is 'Cohort information'
Here you can see that the school has 18% 'Low attainers', 58% 'Middle attainers' and 23% 'High attainers'.
This is an average intake - the middle attainers are those who got level 4 KS2 SATs at primary school (most children), the low attainers got Level 3 or below, and the high attainers got level 5 or above.
Basically:
Low attainers are statistically not going to pass 5 good GCSEs. A few will buck the trend, but the overwhelming majority.
Middle attainers should pass, these are solid C or above students, and should pass, but they are not going to get A*s in most cases
High attainers statistically are nailed on for 5 GCSE passes, and a school that is getting in a lot of high attainers has to be spectacularly fucking awful not to look good in the league tables.
Moving down the list, you see over 80% passing 5 good GCSEs or equivalents including English and Maths. This is very high given the intake.
The next stat is 'Percentage achieving A*-C in English and maths GCSEs'. Maths and English GCSE are they key to the whole league table. Basically the weaker candidates, if they pass this, will then be entered for an easy vocational qualification that is essentially impossible to pass and counts as 4 GCSEs.
As you can see every child who passed English and Maths also got 5 good GCSEs inc. maths and English or (supposedly) equivalent.
As you can see, 99% of the middle attainers did this, as did 50% of the low attainers, and 100% of the high attainers.
The EBacc stats show how many students sat for English, maths, sciences, a foreign language, and a humanity. If your child is a 'high attainer', then you are probably going to want them to follow an academic curriculum, and seeing that most of the students do that is a good sign.
The next figures are 'value added'. This supposedly measures whether the school has added value - i.e. did they take in average students and get them A*s (high value added), or did they take in very bright students and get them Cs (low value added).
I'm very cynical about these because it's obvious that this school is extremely adept at targeting its benchmarks, and you can target 'value added' without necessarily adding value IYSWIM.
Nonetheless, the value added figures are extremely high here, and that is a good sign. 1000 is 'average' here, anything substantially below that indicates the school is worse than average, 1025 would be good, and 1050+ ,as here is outstanding. But like I said, take with a pinch or two of salt.
Next table shows the average grade per GCSE. Ignore average grade per qualification, as it includes the vocational qualifications schools use to make up the 5 GCSEs or equivalent. As you can see, low attainers get a D+ average, middle attainers get a B-, and high attainers get an A average. These grades can be usefully compared between schools - it's possible that School A might appear below School B in the league tables based on the % A*-C GCSE figures, but beat it in every category in terms of average GCSE grade.
Next table is another good one, as it shows the extent to which the school does 'equivalent' qualifications rather than GCSEs (average entries per pupil GCSEs only). As you can see here high attainers sit 11 GCSEs, low attainers 6, middle 9.
A school where middle attainers sit few GCSEs would tend to have a culture of underachievement, and I would worry if I saw that.
For instance this school, by comparison is utterly shit:
www.education.gov.uk/cgi-bin/schools/performance/school.pl?urn=108086
Here you can see that the profile of the cohort is largely identical, and they get in enough bright children at 11 to have several high streams and have them achieve well. But sadly they don't
The data here set alarm bells off all over the place:
45% A*-C inc. English - Maths - this is the headline, and would be enough for parents to avoid the school, but as you scroll down you see failure after failure.
14% of the high attainers don't pass Maths + English (the figure should be 0-1%)
Only 41% of the middle attainers pass maths + English (cf.99% at the outstanding school)
0% of low attainers pass maths + English (cf. 50% at the outstanding school)
Average GCSE grades are crap - B-, D and F cf. A, B- and D+
The children don't appear to be doing any work at all - not only do the 'high attainers' only sit 6.7 GCSEs each, they aren't even making up for it with other qualifications.
A really awful school.
Here's another school, where Nick Clegg has sent his kids to school, and very desirable:
www.education.gov.uk/cgi-bin/schools/performance/school.pl?urn=137157
This school has an admissions policy designed to exclude all but the brightest kids, but without actually testing them. Here you can see that the school takes in 66% High Attainers, 32% Middle Attainers, 2% Low Attainers.
This means that they can essentially tailor their school to the needs of the brightest, because they don't have any academically children to worry about. If your child is very bright, then the high % of high attainers would be a good sign.
Although this school, the Oratory comes above the first (Mossbourne) in the league tables, and middle class parents would generally prefer Oratory, the stats show that Mossbourne is doing a much better job. Because Oratory is stuffed with pushy middle class parents with bright children they don't need to do much to get 90%+ GCSEs, and you see that whereas 99% of 'middle attainers' pass 5 GCSEs at Mossbourne, only 80% (note: this is still very high), do at Oratory.
In addition, Mossbourne is dealing with far worse social problems, high crime rates locally, over 50% on FSMs in past 6 years compared with less than 10% at the Oratory, but they still make damn sure that every child reaches his potential.
Anyway, with specific reference to 53% vs. 81% this comes down to lots of vocational qualifications, but you'd need to drill down into the data to find the fuller picture.