You have to look at the stats for each individual school.
Here's a school that has been failing/in special measures:
www.education.gov.uk/cgi-bin/schools/performance/school.pl?urn=133404
2012 GCSE pass rate = 41%, which is very bad
2013 GCSE pass rate = 60%, which is average nationally, below average for Surrey
Value Added scores are at 1030 (+2SD), which is very good.
However, the number of GCSEs being sat are low:
"Average entries per pupil - all qualifications" - 11.5, 12.5, 13.7 for low, middle and high attainers
"Average entries per pupil - GCSEs only" - 4.0, 5.8, 8.5
Basically the average students are spending the majority of their time on vocational qualifcations (7.7 'entries', which are actually probably much less if you count individual qualifications, as some are counted as equivalent to 2 or 4 GCSEs to only 5.8 entries).
The school is hammering at English and Maths and the average GCSE pass rates are actually very good - E+, C- and B+ respectively, but as you can see from the EB stats (only 10 out of 137; eBacc = English, Maths, science, foreign language and a humanity) and the number of GCSEs sat, the breadth is poor.
Comparing a more desirable middle class school:
www.education.gov.uk/cgi-bin/schools/performance/school.pl?urn=136906
This is a much larger school, with 2000 students, and the result is a large cohort of 'high attainers'/bright pupils (117). GCSE pass grades are similar: E-, C and A-, but the school is more academically focused, with GCSE entries per pupil of 5.6, 9.0 and 10.7, and 138 eBacc entries.
The first school, is basically acting as a form of secondary modern, and actually does better by the lowest ability students than the second one.
Obviously there is much more clamour for a school that does well by the high attainers than the low attainers.