I second all of Burt's excellent recommendations.
For written test preparation, I usually recommend the Pilot Prepware materials from A.S.A. (Aviation Supplies & Academics).
While targeted toward self-study, these materials could certainly be used in a ground school setting.
For audio/video effect, I highly recommend a DVD from Sporty's Pilot Shop entitled "So You Want to Fly Gliders" starring none other than, you guessed it, Bob Wander. This DVD has more glider-related flight/knowledge information packing into a tighter package than any resource I have ever experienced. I have owned it for years and still review it regularly. It is amazing..
For the Oral portion of the practical test, I use and recommend this technique:
Use the Private Pilot Glider Practical Test Standard (PTS) as the study guide and syllabus. As the ground school instructor, work with a glider DPE to develop representative questions for each Objective/Task with each Area of Operation. Document the source(s) of the answers to those questions. These sources then become the library of documents your students should acquire and take with them to the oral. In your ground school sessions, progressively work through the entire PTS, eating the elephant one bite (AoO, Task, Obj.) at a time.
As with any instructional endeavor, the first time through will be the most labor intensive. The results will be worth the effort. One of my students was actually contacted by his DPE after the practical test and told the DPE had never experienced an applicant better prepared for an oral.