they demand to know how student B was punished

I actually was in such a situation as parent A and issued the same demand. The point is: My seven year old son was not concerned about his punishment, he felt and said so, that he did something wrong and there were consequences. But he did not see/hear if stundent B got any punishment. In his mind B therefore did not get any punishment at all and that seemed unfair. The most easy way to get forward in this conversation is to tell him what happened to B. But I do not know. And talking to other parents while on playdates I get told similar problems, the kids don't recognize the "fairnes" of the system if they don't recognize what happens to other offenders. In their mind all the other kids may do forbidden things while not getting punished. The kids of course also lie to each other "I did not get any punishment at all"
We as parents can only say "that kid is lying, of course they got punished" but that's where the conversation ends and the question "What punishment did they get then, eh?" can not be answered.