[jakda]

You can always change your lessons to accommodate those who turn up late, but it isn't fair on the other students. Talk to them individually and explain that it affects others you, and the other students who turn up on time.

They may have reasons for turning up late, but if it is just down to lack of effort to come to class then it isn't fair. It is always harder with adults as calling parents makes a big difference. But beschar offers good advice. Keeping a good relationship within the group makes it harder for those to break the cohesion. Let them know that.