More of the same I see.
Cadet gets a hint they’re getting promoted and tells people, other cadets get upset/annoyed by it and get too excited about it.
This is not a new thing and back in the day when his happened, we used to mock and take the mick out of them, they soon caught on and reeled their neck in.
Is taking the mick and mocking bullying now? If not do some of that, they’ll soon get the message.
Playing devils advocate here, have you thought about it from the Cadet’s point of view?
From your first post you said that he has been told by the staff that he is top of the list for promotion. Sometimed, what you see as arrogance, they see as confidence.
As many have suggested, a conversation is needed with the decision makers so you can air your views, you’ll regret it if you don’t.
But be aware that it might not go well if you come across as being ‘arrogant’ yourself! Make sure you have evidence to back up your views!
Good luck
Since when has this sort of thing been an area up for general discussion?
Would you at work expect your manager to listen or even countenance a chat, if someone was primed for promotion and began to “get a bit big for their boots” and you didn’t like it?
OK it is probably wrong for the staff to prime someone, however the reaction might be their response to how people have treated them previously. If they’ve been treated well by others, no real dramas, not treated well, something like this happens.