It would be a complete no from me, too.
I did my GCSE's a good few years ago, now - I'm in my early 20's. I was a good kid, and I enjoyed studying. I tended to put extra reading/work in anyway, and thrived on teaching my younger sisters what I'd learnt, which probably helped me to retain knowledge.
There were plenty of able people in my class without that motivation, though, and they required a high standard of teaching and care to get a pass. They were perfectly capable of doing it, but it needed to be well covered in each class, so that no extra work was needed.
I'd be concerned that this school won't cover everything in an adequate way, and therefore anyone who does need additional pushing or encouragement is at a disadvantage. A lot of those people who needed it from my class are now teachers, lawyers, doctors. We're a high-achieving bunch, nobody would argue that they weren't capable of passing.
I don't think it is true that the 52% wouldn't get an A-C wherever they go. It means that they aren't being appealed too in a way that works for them. It's lazy thinking to believe that they just can't do it - even if there are a certain percentage of people who just won't be able to achieve that standard, it's not 52%.
If you're torn, it's still worth visiting the school, and seeing what you think. Or sending your children, but having a back up plan that you can quickly implement if they don't achieve their potential.