I know a way to make it stop mc: tell the class outright that you saw the chat and that it's all lies. Then address specific things such as "regarding that claim that she takes cash, anyone who can actually verify it? Nope? Well because that's not true." or "hey kirishima, one message claimed you had stated to them that you refused to pay her. Did she ever ask you to pay? Why would you have told someone such a lie that they now spread onwards?" (likely he never did make such a lie, but if he is innocent he will clear it all up, and if not then you know your culprit and can point at them as a jealous bastard).
And ask the ones who made the claims in the chat that they saw stuff to identify themselves as the posters. Because at the moment it just seems like slander by people who most likely has a grudge against them or friends to such a person.
Maybe also ask people to identify who said what by having them post some secret number they are given in person (assuming this was the type of anonymous chat joined using their line-id's, and not one where ppl can making multiple accounts to slander them), to find the poster's through deduction by exclusion.
Japanese bullying, I see it depicted all the time, but I never saw anything comparably vicious over here.
Mostly because that kind of thing is too easy to stop. Make a stink about it and the origin will either be exposed, or teachers and parents will step in and stop it. Teachers might fail at first, but when they bring parents in the phones starts being looked at and bullies identified. Hell, cops can be involved at a certain point, and then all this is just proof waiting to be used.
I imagine we will see this kind of thing for real soon though, as kids become more savvy in how to properly keep things anonymous due to it becoming both easier to do (You need to create throwaway accounts to set everything up and give everyone access to join through their own throwaways. You also need to keep everything done either through the cloud with throwaway accounts to connect - so no evidence is left on your physical devices - or you need to encrypt stuff, but that is sus) and easier to find out how to do.
