is this ok and if so what would you tell immigration officials and how would you fill in immigration card etc when she first gets there?

Personally, I don't think it's a good idea. The rules on the HK Immigration Department say clearly "those who travel in HK shall not become a student at a school, university or other educational institution." If no one reports your daughter to the related department, everything is ok. What if your daughter was found to study when she "travelled" in HK?