In a specialized context (such as among people who work at a hospital), "patient's relative" could conceivably become a set phrase, in which case sentences such as "there are some patient's relatives waiting outside" would become possible (contrast * "there are some Greg Egan's stories on the shelf").

This is presumably what happened with "girls' school". Very rarely, it can even happen with proper nouns, as in the mathematical term Green's function. But this is not part of the syntax of the possessive ; it is the result of the whole possessive phrase being treated as a unit. (When you hear "the Green's function for this operator" for the first time, you immediately know that "Green's function" is a jargon phrase, because of the irregular syntax.)