I studied in St Catherine's School in the UK before and also went to NTK for my GCSE summer courses. I must say they've quite good teachers and also ask you for some detailed academic background in order to tailor-make the syllabus for you. So I will definitely recommend (:
My friend's children study in South Island School and they also recommend NTK to me. I know they are good but the fee also very high, so I still consider. Anyway, I will make an appointment with NTK when my son back to HK.