We waited 6 years to get our daughter into DBIS! We registered her when she was 6 months old, and she was finally accepted into P3.
What did us in was the introduction of the "premium debenture" scheme, which for a non-refundable payment of HKD100K (per child), children were allowed to "jump the queue" and go to the top of the waiting list.
The school debenture system really sucks. Debentures (traded on the open market) reached unbelievable levels last year. Chinese locals, desperate to keep their kids out of local schools, were paying debentures of up to HKD3 MILLION for some schools (South Island School, Canadian International School and others).
It's a poorly disguised form of extortion which has you by the short&curlies unless you cough up.