I recently picked up the music book, "Jazzed up! Classics" by Kevin Olson.
I LOVE IT! Of course, I love just about anything done by Kevin Olson. This book is a real gem. Now if you are a classical music purist, this may not be for you, but I found it delightful. Kevin took 7 classical pieces and well, "jazzed them up!". Some of them are in jazz style while others have a more rock or pop sound. They are extremely well done and I found them truly fun to play. They are written for the late intermediate student, but are still fun for the advanced player.
I especially loved "Lula-blue" - a jazzed up version of Brahms' Lullaby. It is sweet and tender. A couple of others I really enjoyed are "Springtime Boogie" - a swing version of Vivaldi's "Spring" from The Four Seasons - and "A Joyful Ode" - a nice arrangement of Beethoven's "Ode to Joy". Each piece in the book is well done and very creative.
This book would make a nice addition to your studio and could easily work as an example of improvisation with your students! It may even encourage them to give it a try!