I use Pencil with my students. It's such an easy program to teach and the kids pick up on it so quickly! I love that it's free (for both PC and Mac) because it gives them the option to download it at home. I have my students export their animations to .MOV files which they use in iMovie as part of a larger project. How do you like working with Toon Boom? Would you compare it to Flash? I think it might it be too much for 8th grade.
I agree that Pencil is great: free / cross- platform / easy-to-use.
Toon Boom is a stretch because of the learning curve but I have a friend who uses it with H.S. students and has a lot of success. I jsut bought some tutorials and I plan to use it at a summer camp with the older, experienced kids.