I am not familiar with HK law but much of it follows British law and under British law you have no rights because photographs are not covered by personal privacy rights. There are data privacy rights, privacy acts that govern what companies can collect about say your financial information or health information. But there's no right to say 'This is my privacy, do not take a photo of me'.
There are two distinct situations, one regarding public space/place and the other regarding private property.
It is legal to take a photograph of anyone in a public space: there are no laws that prohibit such activity (unless trespass is involved), and the subject has no jurisdiction over the act by claiming the right to state that: This is my privacy, you cannot take a photo of me. In the case of someone on the street taking a photograph, as long as they're not physically trespassing on your land (or someone elses land who objects to their presence), there's not really any law that would come into play there to prevent that.
In private, non-public spaces/places, the owner (in this case the lessee) has the right to set conditions under which photographs maybe taken, or not. For example, at the Saturday gym class my grand-daughters attend in England, no photographs are allowed to be taken of the children inside the building. But if the owner approves someone to take photos then that is legal, and the images will belong to the photographer or, if taken at the direction of another party, by that agency (in this case the school) under copyright law. Copyright is outside your legal jurisdiction, the subject (you) has no legal rights to the contrary, and the photographer has the right to publish their photos wherever they like (pornographic images and child exploitation images excepted). There is very little that can be done to stop photographs being taken where no-one is compromised in this way.
Ethical considerations come into the frame however, depending upon the use to which photographs are put. Legal considerations may come into play usually and only if your image is used without your permission for commercial gain by a third party, or if you are in some way defamed.
In this case it would appear to be a concern ethically, and one would suppose that a private school would want to avoid adverse publicity by posting photos of people against their will, even if those photos were taken on their premises under an approved arrangement with the photographer.
I would write politely (not angrily) direct to the principal copied to the Chairman of the Board, expressing your concern, noting disappointment/dismay at the attitude of the manager, stating that you assume that the school would want to avoid adverse publicity by retaining the photo on their web site against your wishes, and request that they take it down. You could acknowledge that there is no legal impediment to the school keeping it on their web site but that ethically it is questionable given your objection and as such probably not in line with the schools standards of honour.