Math or Maths?

It's 'Math' in American English and 'Maths' in British English. So, both are correct.

Hong Kong generally uses British English so you study 'Maths' here.

Here are some more examples of spellling differences between British English (BrE) and American English (AmE):

1. centre (BrE) / center (AmE)
   
   
2. litre (BrE) / liter (AmE)
   
   
3. colour (BrE) / color (AmE)
   
   
4. neighbour (BrE) / neighbor (AmE)
   
   
5. catalogue (BrE) / catalog (AmE)
   
   
6. dialogue (BrE) / dialog (AmE)
   
   
7. travelling (BrE) / traveling (AmE)
   
   
8. cancelled (BrE) / canceled (AmE)
   
   
9. skilful (BrE) / skillful (AmE)
   
   
10. enrolment (BrE) / enrollment (AmE)
    
    

Are you able to deduce some rules about American spelling and British spelling from the above?