Hi there Liney, I did not go to Chatham Boys open evening however I did attend several other grammars and one high school who also follow this accelerated curriculum. The way it was explained was that it is extremely unlikely a future employer/university will be greatly interested in a childs SATs results, however they will definitely be very interested in their GCSE results. Because of this more and more schools have decided not to devote 3 years to SATs work but cover it in 2 years leaving more time to really nail the GCSE's. If a pupil does badly they can retake it in year 11 or even pick up a new one.
I do understand your concerns however, but it seems most of the schools are doing this now. Hope this was useful.