Potentially affected data may include contact details and online shopping order information, while police reported six impersonation scams causing more than HK$300,000 in losses.
The school has returned to one site after its former Wang Fuk Court premises were decommissioned following a fire and pupils spent a period studying at two other district schools.
Education Secretary Christine Choi says mergers and other gradual measures will help schools adjust to a structural decline in the school-age population.
Activity reached or slightly passed a peak in August, but health officials say it is too early to estimate how long the period of heightened activity will last.