HKIS give priority to U.S. passport holder. They will also guarantee spot for debenture holder.
CIS will give you a chance to be tested if you buy a debenture. Not guarantee acceptance.
CDNIS give priority to Canadian passport holder & debenture holder.
Also there is German Swiss International School, French International School, Singapore International School, ESF Schools(multiple schools). Besides GSIS, the rest are easier than the top 3.
Overall HK has a shortage in International Schools. Lot of expats have to leave their children in home country because they can't get in school.