I've had a brief look at the ICAC and Eduation Bureau website, and it seems that it is up to the School Management Committee to formulate their own policies, bearing in mind the bribery ordinance.
There is however a 'sample code of conduct' on the Education Bureau website which states:
Accepting Advantages
5. Permission is given for staff to accept, but not solicit, the following advantages: -
(a) gifts from pupils or parents, provided that the value of the gift does not exceed ($100);
(b) gifts from pupils or parents at graduation ceremonies provided that the aggregate value of the gifts does not exceed ($500) on each occasion;
(c) gifts from parents, colleagues, pupils or former pupils of the School on the occasion of retirement or resignation provided that the value of the gift from any one person does not exceed ($1,000).
[Note: Figures in brackets are considered realistic and recommended by the Education Bureau for adoption by schools.]