It was a group interview with around 6 kids,the teachers brought them to classroom and parents are not allowed to go together, my son told me that he played the toys there, read the books and the teacher asked him to draw something,maybe chatted with the kids during the interview. The interview was around an hour.Hope this can help.
Actually the principal told us about the no. of classes before, seems 2 reception classes, you may take a school tour and ask about the details.
My son is also holding HK passport and he is accepted, so don't worry