Don't remember if there were a portrait in every class room but the queen portraits were every where. My sisters attended Protestant and Catholic schools and there are more brainwashing and whitewashing in those schools.
There you go. You remembered the queens portrait then. So there shouldn't be anything wrong with Hong Kong schools having Chinese flags or flag raising ceremonies. Etc.
I know how your sister felt. I went to a protestant school for one year. Basically because my mother was protestant due to her dad converted. So my mother decided to take advantage of concessions because of affordability. I can tell you I hated it.