Neil Mercer has done a lot of research on group discussion in classrooms and has found that simply asking students to work together in groups is often a waste of time. Students need to have opportunities to learn how to talk together productively in groups. Here is Professor Mercer making a plea to schools to teach pupils how to have reasoned discussions together:
A quick way to get started is to simply ask your students what a good discussion looks like, and also what a bad discussion looks like! Giving them the opportunity to think and talk about this is a gamechanger.
Notes from an Oracy INSET – teachers thinking about what a bad discussion looks like
Learners recall fiction, processes, explanations and more by listening and building on what others have said

Get them really listening to, and thinking about, each other’s contributions