What metaphors are used in Master Harold and the Boys by Athol Fugard?

"You sure of that, Hally? Because it would be pretty hopeless if that was true. It would mean nothing has been learnt in here this afternoon, and there was a hell of a lot of teaching going on . . . one way or the other. But anyway, I don't believe you. I reckon there's one thing you know. You don't have to sit up there by yourself. You know what that bench means now, and you can leave it any time you choose. All you've got to do is stand up and walk away from it."

Despite everything that has happened, Sam remains optimistic and gives Hally -- whom he is calling by his nickname again -- an opportunity to be a better man
through the metaphor of the whites-only bench.