The way to keep a conversation private is to use the DM facility of Discourse (“Message”). You can send a private message to a single person or to a list of people.
There is one complicating factor here: there seems to be some kind of platform bug right now that causes problems with the quiz answers getting scrambled and not correctly matching the questions. Not sure if that is what is happening in your particular case, but something to keep in mind. Here’s a thread about this issue.