Hi mummydd,
DS went to CCS for years 7 & 8 before we moved away from Finchley last year.
I have rather mixed feelings about the school, but DS had some specific problems which have clouded the issue for us.
Considering the pupil intake and location the school performs extremely well academically. 23 boys from DS's primary school went there and the overwhelming majority of parents and kids are happy with the school. They get very good GCSE results - 76% passed 5 subjects including English and Maths this year. 6th form results not so stellar, but the most academic pupils are creamed off by Woodhouse College for A levels.
Just for comparison, DS's current school is also a boys comprehensive and former grammar school with around 700 pupils and a co-ed 6th form. But the parents are (according the latest Ofsted) predominantly white middle class professionals, and there are fewer than average pupils eligible for free school meals. It is hugely oversubscribed with a significant number of pupils switching from the private sector to go there. It still has a lower GCSE pass rate than CCS.
If your DS is sporty, sociable and reasonably resilient I'm sure he'd thrive at CCS. It has a lively and mostly positive atmosphere.
The school manages to keep a lid on the worst types of bad behaviour and has a very clear and effective system of rewards and incentives for good behaviour (merit and house points rewarded with certificates, trips to theme parks etc).
There is however a widepread tolerance of low-level disruption in classes, and out-of class pupil behaviour (including out of school) can be very boisterous.
The school buildings are rather tired, "overcrowded, functional and utilitarian" is the kindest way I can think to describe them. There is very little extra-curricular music or drama provision, but lots of sports clubs and CCF.