I've done a few MS drama CA classes. One of the biggest obstacles is students of different ages in the same class. Any time you're trying to come up with ideas the older ones dominate the discussion and the younger ones contribute nothing. It might even be better to split their work by age groups.
The biggest motivating factor is a performance in front of the school. Nothing motivates kids more than not wanting to look stupid in front of the whole school. Find a school event where you could perform a short skit or some skits and get the kids working on preparing for that. If they're musically inclined you could try a scene from a musical (Grease or HS Musical would work well) or you could find a different drama and tailor it to the students you have. Skits set in a school setting always work well.