Here are some more examples about it:
1. a school for girls => a girls' school
2. a magazine for women => a women's magazine.
3. Charles's room
I guess, the Indefinite Article from the first example is addressed to the noun in the Possessive Case girls', isn't it? If so, then we use the Indefinite Article before singular noun girls'. Is it correct?
Should we add 's or simply ' in the third example? If the given example is correct, should we then read it putting iz at the end?