It's a little disingenuous to use the word "schoolmates" to describe his fellow prisoners. This was at Berkeley; as far as I'm aware, none of the guards knew the prisoners before the experiment.

Honestly, if it weren't unethical, I'd be curious as to the results of a re-run of the SPE except that you spend the first day out-of-character doing icebreakers before role assignment. I suspect that it'd make things better on the whole, but you'd have the occasional case where a guard uses it as an occasion to fish for ways to get under a prisoner's skin much more quickly and effectively.