We take the approach DeeDee takes - we help talk our kids to a conclusion of what would they do if (insert fear) did happen? Having a coping strategy helps reduce their anxiety. I also wouldn't tell them the mean kids won't be in his class... unless you know absolutely for sure they won't. I might tell him there's a good chance they won't be in his class, and that I hope they won't be in his class (or that I hope the kids in his class won't be mean this year... not exactly the same thing!).
I'm curious if you have any idea why he's worried that the work will be too hard? Is it because he knows the class is going to be leveled? Is it just because it's another grade higher? Has he had trouble with a specific type of classwork in the past? Is he a perfectionist?
polarbear