Don’t be confused by the title. There is no evidence for anyone calling it The School of Athens before 1671 and the first reference to it as that in print dates from 1685. (Indeed, some early commentators, including Vasari, didn’t even think that it showed a group of Greek philosophers.) Once you realise that Raphael never called it by its later name, it becomes easier to recognise that he didn’t necessarily intend it to be anywhere in particular.
Also, as well as the points others have mentioned, there’s the fact that the Stanza della Segnatura is vaulted, so the barrel vaults in the background not-so-subtlely pick up on that semi-circular emphasis.