I think what we sometimes miss is that a good flipped activity should address the topics of the work students are expected to do in advance, just at a higher level. This removes the complaint of the students that information was not covered in the classroom. By engaging in the classroom activity, they will still be exposed to the content covered, but those who have failed to prepare may struggle compared to their peers. When the classroom activity is a group activity, peers who have prepared can influence their group mates to prepare better in the future.
