When we have pieces that are below the technical level of the group, but still need drilling (I'm doing some film music now) I'll start at the last rehearsal number and go to the end until they've got a grasp on it, then work backwards. I've found their more motivated to keep going towards the beginning than they are to keep drilling towards the end. Good results in the concert as well, as the piece gets really polished at the end. The other poster is right, that students will always start practicing the beginning, so that takes care of itself.