Sometimes HK people just don't appreciate how lucky they are, they can send their kids to their international school of choice. Many countries do not allow local kids to study in IS.
The more bizzare a thing is, the less mysterious it proves to be.
Go and argue that with Chinese government or the Singapore government. Or any number of government who forbid local citizens enter IS.
IS are not just private schools. Comparing Government hospital to private hospital is not appropriate.
Because IS are meant to serve non locals education need so foreigners can relocate to HK to work. And there are conditions to observe (70%) when the school was granted land to set up the school.
Don't get me wrong I am not saying HK should not have local students, I am saying people should "appreciate" the freedom we have, and stop saying the government should do this and that and suggest that 30% local may not be reasonable in IS.