Make sure your students understand that it's not easy, they're going to suck early on, and it's going to hurt (physically). Explain calous and strength building, ect.
-Metaellihead
Just a few suggestions -
If you've got a large group, for a few lessons, after you've got to know them, you could divide them up into groups and try out having a little band practice, with some playing lead and others playing rhythm, to make it fun, and help them to learn the idea of keeping time in a group.
As an incentive, if they've played well, you could give each a turn to bring in their favourite guitar track to be played at the end of the lesson. (That is one or two tracks per week, so they all get a turn, but not too many so they think it's a bludge)
Try and teach them over the course of the time a simplified version of one of their favourite songs.
Plan and be organized, and have something up your sleeve in case you've got extra time left in a particular lesson. Practise in front of your family giving the class, they can advise you how well you are coming across .
Good luck, and please let us all know how you get on.
