Levels is a plug-in I’ve never had a problem with and my students seem to grasp it really well. Curves, however, is another animal. My students sometimes have a tough time figuring out how it works. Maybe we’ll see a reference guide for curves? Heck, maybe I will make one.
Update: John tells me that he IS working on a Curves reference guide. Thanks, John! I know that will certainly be used in my classroom.