Well, why would anyone risk failing just for the chance that everyone passes? Just because that one guy one gets a black stone? Who cares, as long as I pass? Only a idiot would deliberatly put down the fail-the-class stone if they don't have to.
Nah, I would have placed the black one as well. And not just because I would be in one room with Alison in this scenario. But I'm pretty sure (some of) the students that choose a white stone thought something like that. A shame not all of them follow Alison's axiom. I mean seriously? Only Al put down a black stone? That is just sad. I hope there are a few others offscreen.