The card costs HK $50 when you apply for a new one. 

You can visit the IT Office in D710 and apply for a new one.  You will be asked to fill up a form and pay the $50 using Octopus payment

Printing and processing of the new card takes about a couple of hours only.   If you apply for it in the morning then you will be asked to collect it during lunchtime.
If you apply for it during lunchtime or afternoon, you will be asked to collect it the next day.

Yes, you need to return it.  If you don't return it, you won't be able to apply for a refund if you have more than HK $25 printing credit balance.  Below HK $25 printing credit balance will be automatically donated to school.