Hi! As Tdol says it would be easier if you give us more details.
If the problem is ability and they are just a few sts, they may work in smaller groups. Try to test them to see their levels so you can divide the big group. It's harder for the teacher but it gives excellent results. This I suggest gradually, with fewer groups, so after the time you think it appropiate they will be almost at the same level.
If the problem is attitude, I suggest
a) making THEM choose/vote the topics or means they like. Based on them you can plan a lesson with an article/text/letter/song/etc. with the grammar point or vocabulary you want to teach. (I remember using basketball players to teach biographies)
b) Timing: give exact time to do the exercise, so they can't delay the task.
c) Be nice but firm. This took me ages to acquire!:-D