A couple of my favorites include:
It was a pleasure getting to know you and have you as a teacher. I learned a lot and I hope the next group of kids enjoy you as much as we did. I'm sorry if I was rude sometimes. Good Luck!
Best wishes to your success in your future. I appreciate all your help.
I had lots of fun having you as a teacher. Good luck at your next school!
It was great having you teach us and getting to know you! Best of Luck!
(And for the really poignant ones..)
You have taught me well.
Thanks for the learning stuff.
I had to snap some photos with my art ladies... I sure will miss my students.