A teacher I know would say to have your slides ready to go and well rehearsed to show your teacher that you did your share, along with a basic outline of what they should have done. Maybe have a few blank slides with what would have been their titles, ready to fill in. Any good teacher should realize that one responsible student can't be liable for the others in the group and should be willing to adjust the scores as needed. Just be up front with the teacher and say the others didn't come through. Of course, some teachers will hold everyone in the group responsible for everything, so it'll have to be your judgement call.