KenKen is a brilliant resource for fostering numeracy. Please do yourself a favor and never give grades for Kenken (unless for "extra credit"). Standardization and grading could ruin a wet dream, imagine what they could do to a fun math resource!
KenKen is easy to learn, but hard to "master" (if that's even possible).
There are so many people who never strive for a higher level of play than they are at, for various reasons. One of them is that if you're not in some sort of community that can help you along (like a forum, or a convention), there isn't much opportunity to see advanced play as it as done.
To help remedy that, I've created about 30 videos than anyone can view at mathmojo.com/kenken, which walk you through a 9x9 KenKen without operations signs.
The series is not made to be an ultimate kind of course, though. They are basically to be a "hand-holding" for people who are not sure if they could tackle something like that. You know that old phenomenon - you might know all the mechanics of doing something, but until you see it done, step-by-step, you may lack the confidence of tackling one yourself.
I was hoping that once people see that most "hard" kenken need not really be that hard, once you break them down, they will feel a little more able of working on them along.
KenKen can be addictive. I think, as a teacher, you will see wonderful results, especially from some of the students who used to "hate math."
Have fun, and go figure....
Brian (a.k.a. Professor Homunculus at MathMojo.com )